The putnam mathematica competition 2001 2008 AMS

The 62nd William Lowell Putnam Mathematical Competition Saturday, December 1, 2001 A-1 Consider a set and a binary operation , i.e., for each , . Assume for all . Prove that for all . A-2 You have coins . For each , is bi- ased so that, when tossed, it has probability of fallings heads. If the coins are tossed, what is the probability that the number of heads is odd? Express the answers as a rational function of . A-3 For each integer , consider the polynomial For what values of is the product of two non- constant polynomials with integer coefficients? A-4 Triangle has an area 1. Points lie, respec- tively, on sides , , such that bisects at point , bisects at point , and bisects at point . Find the area of the triangle . A-5 Prove that there are unique positive integers , such that . A-6 Can an arc of a parabola inside a circle of radius 1 have a length greater than 4? B-1 Let be an even positive integer. Write the numbers in the squares of an grid so that the -th row, from left to right, is Color the squares of the grid so that half of the squares in each row and in each column are red and the other half are black (a checkerboard coloring is one possi- bility). Prove that for each coloring, the sum of the numbers on the red squares is equal to the sum of the numbers on the black squares. B-2 Find all pairs of real numbers satisfying the sys- tem of equations B-3 For any positive integer , let denote the closest in- teger to . Evaluate B-4 Let denote the set of rational numbers different from . Define by . Prove or disprove that where denotes composed with itself times. B-5 Let and be real numbers in the interval , and let be a continuous real-valued function such that for all real . Prove that for some constant . B-6 Assume that is an increasing sequence of pos- itive real numbers such that . Must there exist infinitely many positive integers such that for ? This is trial version www.adultpdf.com The 63rd William Lowell Putnam Mathematical Competition Saturday, December 7, 2002 A1 Let k be a fixed positive integer. The n-th derivative of 1 x k −1 has the form P n (x) (x k −1) n+1 where P n (x) is a polyno- mial. Find P n (1). A2 Given any five points on a sphere, show that some four of them must lie on a closed hemisphere. A3 Let n ≥ 2 be an integer and T n be the number of non- empty subsets S of {1, 2, 3, . . . , n} with the property that the average of the elements of S is an integer. Prove that T n − n is always even. A4 In Determinant Tic-Tac-Toe, Player 1 enters a 1 in an empty 3 × 3 matrix. Player 0 counters with a 0 in a va- cant position, and play continues in turn until the 3 × 3 matrix is completed with five 1’s and four 0’s. Player 0 wins if the determinant is 0 and player 1 wins other- wise. Assuming both players pursue optimal strategies, who will win and how? A5 Define a sequence by a 0 = 1, together with the rules a 2n+1 = a n and a 2n+2 = a n + a n+1 for each inte- ger n ≥ 0. Prove that every positive rational number appears in the set  a n−1 a n : n ≥ 1  =  1 1 , 1 2 , 2 1 , 1 3 , 3 2 , . . .  . A6 Fix an integer b ≥ 2. Let f(1) = 1, f(2) = 2, and for each n ≥ 3, define f(n) = nf(d), where d is the number of base-b digits of n. For which values of b does ∞  n=1 1 f(n) converge? B1 Shanille O’Keal shoots free throws on a basketball court. She hits the first and misses the second, and thereafter the probability that she hits the next shot is equal to the proportion of shots she has hit so far. What is the probability she hits exactly 50 of her first 100 shots? B2 Consider a polyhedron with at least five faces such that exactly three edges emerge from each of its vertices. Two players play the following game: Each player, in turn, signs his or her name on a previously unsigned face. The winner is the player who first succeeds in signing three faces that share a common vertex. Show that the player who signs first will always win by playing as well as possible. B3 Show that, for all integers n > 1, 1 2ne < 1 e −  1 − 1 n  n < 1 ne . B4 An integer n, unknown to you, has been randomly chosen in the interval [1, 2002] with uniform probabil- ity. Your objective is to select n in an odd number of guesses. After each incorrect guess, you are informed whether n is higher or lower, and you must guess an integer on your next turn among the numbers that are still feasibly correct. Show that you have a strategy so that the chance of winning is greater than 2/3. B5 A palindrome in base b is a positive integer whose base- b digits read the same backwards and forwards; for ex- ample, 2002 is a 4-digit palindrome in base 10. Note that 200 is not a palindrome in base 10, but it is the 3- digit palindrome 242 in base 9, and 404 in base 7. Prove that there is an integer which is a 3-digit palindrome in base b for at least 2002 different values of b. B6 Let p be a prime number. Prove that the determinant of the matrix   x y z x p y p z p x p 2 y p 2 z p 2   is congruent modulo p to a product of polynomials of the form ax + by + cz, where a, b, c are integers. (We say two integer polynomials are congruent modulo p if corresponding coefficients are congruent modulo p.) This is trial version www.adultpdf.com The 64th William Lowell Putnam Mathematical Competition Saturday, December 6, 2003 A1 Let be a fixed positive integer. How many ways are there to write as a sum of positive integers, , with an arbitrary positive integer and ? For example, with there are four ways: 4, 2+2, 1+1+2, 1+1+1+1. A2 Let and be nonnegative real numbers. Show that A3 Find the minimum value of for real numbers . A4 Suppose that are real numbers, and , such that for all real numbers . Show that A5 A Dyck -path is a lattice path of upsteps and downsteps that starts at the origin and never dips below the -axis. A return is a maximal sequence of contiguous downsteps that terminates on the -axis. For example, the Dyck 5-path illustrated has two re- turns, of length 3 and 1 respectively. O Show that there is a one-to-one correspondence be- tween the Dyck -paths with no return of even length and the Dyck -paths. A6 For a set of nonnegative integers, let denote the number of ordered pairs such that , , , and . Is it possible to partition the nonnegative integers into two sets and in such a way that for all ? B1 Do there exist polynomials such that holds identically? B2 Let be a positive integer. Starting with the sequence , form a new sequence of entries by taking the averages of two con- secutive entries in the first sequence. Repeat the aver- aging of neighbors on the second sequence to obtain a third sequence of entries, and continue until the final sequence producedconsists of a single number . Show that . B3 Show that for each positive integer n, (Here denotes the least common multiple, and denotes the greatest integer .) B4 Let where are integers, . Show that if is a rational number and , then is a rational number. B5 Let , and be equidistant points on the circumfer- ence of a circle of unit radius centered at , and let be any point in the circle’s interior. Let be the dis- tance from to , respectively. Show that there is a triangle with side lengths , and that the area of this triangle depends only on the distance from to . B6 Let be a continuous real-valued function defined on the interval . Show that This is trial version www.adultpdf.com The 65th William Lowell Putnam Mathematical Competition Saturday, December 4, 2004 A1 Basketball star Shanille O’Keal’s team statistician keeps track of the number, S(N), of successful free throws she has made in her first N attempts of the sea- son. Early in the season, S(N) was less than 80% of N, but by the end of the season, S(N) was more than 80% of N. Was there necessarily a moment in between when S(N) was exactly 80% of N? A2 For i = 1, 2 let T i be a triangle with side lengths a i , b i , c i , and area A i . Suppose that a 1 ≤ a 2 , b 1 ≤ b 2 , c 1 ≤ c 2 , and that T 2 is an acute triangle. Does it follow that A 1 ≤ A 2 ? A3 Define a sequence {u n } ∞ n=0 by u 0 = u 1 = u 2 = 1, and thereafter by the condition that det  u n u n+1 u n+2 u n+3  = n! for all n ≥ 0. Show that u n is an integer for all n. (By convention, 0! = 1.) A4 Show that for any positive integer n, there is an integer N such that the product x 1 x 2 · · · x n can be expressed identically in the form x 1 x 2 · · · x n = N  i=1 c i (a i1 x 1 + a i2 x 2 + · · · + a in x n ) n where the c i are rational numbers and each a ij is one of the numbers −1, 0, 1. A5 An m × n checkerboard is colored randomly: each square is independently assigned red or black with probability 1/2. We say that two squares, p and q, are in the same connected monochromatic component if there is a sequence of squares, all of the same color, starting at p and ending at q, in which successive squares in the sequence share a common side. Show that the expected number of connected monochromatic regions is greater than mn/8. A6 Suppose that f(x, y) is a continuous real-valued func- tion on the unit square 0 ≤ x ≤ 1, 0 ≤ y ≤ 1. Show that  1 0   1 0 f(x, y)dx  2 dy +  1 0   1 0 f(x, y)dy  2 dx ≤   1 0  1 0 f(x, y)dx dy  2 +  1 0  1 0 (f(x, y)) 2 dx dy. B1 Let P (x) = c n x n + c n−1 x n−1 + · · · + c 0 be a poly- nomial with integer coefficients. Suppose that r is a rational number such that P (r) = 0. Show that the n numbers c n r, c n r 2 + c n−1 r, c n r 3 + c n−1 r 2 + c n−2 r, . . . , c n r n + c n−1 r n−1 + · · · + c 1 r are integers. B2 Let m and n be positive integers. Show that (m + n)! (m + n) m+n < m! m m n! n n . B3 Determine all real numbers a > 0 for which there ex- ists a nonnegative continuous function f (x) defined on [0, a] with the property that the region R = {(x, y);0 ≤ x ≤ a,0 ≤ y ≤ f(x)} has perimeter k units and area k square units for some real number k. B4 Let n be a positive integer, n ≥ 2, and put θ = 2π/n. Define points P k = (k, 0) in the xy-plane, for k = 1, 2, . . . , n. Let R k be the map that rotates the plane counterclockwise by the angle θ about the point P k . Let R denote the map obtained by applying, in order, R 1 , then R 2 , . . . , then R n . For an arbitrary point (x, y), find, and simplify, the coordinates of R(x, y). B5 Evaluate lim x→1 − ∞  n=0  1 + x n+1 1 + x n  x n . B6 Let A be a non-empty set of positive integers, and let N(x) denote the number of elements of A not exceed- ing x. Let B denote the set of positive integers b that can be written in the form b = a − a  with a ∈ A and a  ∈ A. Let b 1 < b 2 < · · · be the members of B, listed in increasing order. Show that if the sequence b i+1 − b i is unbounded, then lim x→∞ N(x)/x = 0. This is trial version www.adultpdf.com The 66th William Lowell Putnam Mathematical Competition Saturday, December 3, 2005 A1 Show that every positive integer is a sum of one or more numbers of the form 2 r 3 s , where r and s are nonneg- ative integers and no summand divides another. (For example, 23 = 9 + 8 + 6.) A2 Let S = {(a, b)|a = 1 , 2, . . . , n, b = 1, 2, 3}. A rook tour of S is a polygonal path made up of line segments connecting points p 1 , p 2 , . . . , p 3n in sequence such that (i) p i ∈ S, (ii) p i and p i+1 are a unit distance apart, for 1 ≤ i < 3n, (iii) for each p ∈ S there is a unique i such that p i = p. How many rook tours are there that begin at (1, 1) and end at (n, 1)? (An example of such a rook tour for n = 5 was depicted in the original.) A3 Let p(z) be a polynomial of degree n, all of whose ze- ros have absolute value 1 in the complex plane. Put g(z) = p(z)/z n/2 . Show that all zeros of g  (z) = 0 have absolute value 1. A4 Let H be an n × n matrix all of whose entries are ±1 and whose rows are mutually orthogonal. Suppose H has an a × b submatrix whose entries are all 1. Show that ab ≤ n. A5 Evaluate  1 0 ln(x + 1) x 2 + 1 dx. A6 Let n be given, n ≥ 4, and suppose that P 1 , P 2 , . . . , P n are n randomly, independently and uniformly, chosen points on a circle. Consider the convex n-gon whose vertices are P i . What is the probability that at least one of the vertex angles of this polygon is acute? B1 Find a nonzero polynomial P (x, y) such that P (a, 2a) = 0 for all real numbers a. (Note: ν is the greatest integer less than or equal to ν.) B2 Find all positive integers n, k 1 , . . . , k n such that k 1 + · · · + k n = 5n − 4 and 1 k 1 + · · · + 1 k n = 1. B3 Find all differentiable functions f : (0, ∞) → (0, ∞) for which there is a positive real number a such that f   a x  = x f(x) for all x > 0. B4 For positive integers m and n, let f (m, n) denote the number of n-tuples (x 1 , x 2 , . . . , x n ) of integers such that |x 1 |+ |x 2 |+ · · · +|x n | ≤ m. Show that f(m, n) = f(n, m). B5 Let P (x 1 , . . . , x n ) denote a polynomial with real coef- ficients in the variables x 1 , . . . , x n , and suppose that  ∂ 2 ∂x 2 1 + · · · + ∂ 2 ∂x 2 n  P (x 1 , . . . , x n ) = 0 (identically) and that x 2 1 + · · · + x 2 n divides P(x 1 , . . . , x n ). Show that P = 0 identically. B6 Let S n denote the set of all permutations of the numbers 1, 2, . . . , n. For π ∈ S n , let σ(π) = 1 if π is an even permutation and σ(π) = −1 if π is an odd permutation. Also, let ν(π) denote the number of fixed points of π. Show that  π ∈S n σ(π) ν(π) + 1 = (−1) n+1 n n + 1 . This is trial version www.adultpdf.com The 67th William Lowell Putnam Mathematical Competition Saturday, December 2, 2006 A1 Find the volume of the region of points (x, y, z) such that (x 2 + y 2 + z 2 + 8) 2 ≤ 36(x 2 + y 2 ). A2 Alice and Bob play a game in which they take turns removing stones from a heap that initially has n stones. The number of stones removed at each turn must be one less than a prime number. The winner is the player who takes the last stone. Alice plays first. Prove that there are infinitely many n such that Bob has a winning strat- egy. (For example, if n = 17, then Alice might take 6 leaving 11; then Bob might take 1 leaving 10; then Alice can take the remaining stones to win.) A3 Let 1, 2, 3, . . . , 2005, 2006, 2007, 2009, 2012, 2016, . . . be a sequence defined by x k = k for k = 1, 2, . . . , 2006 and x k+1 = x k + x k−2005 for k ≥ 2006. Show that the sequence has 2005 consecutive terms each divisible by 2006. A4 Let S = {1, 2, . . . , n} for some integer n > 1. Say a permutation π of S has a local maximum at k ∈ S if (i) π(k) > π(k + 1) for k = 1; (ii) π(k − 1) < π(k) and π(k) > π(k + 1) for 1 < k < n; (iii) π(k −1) < π(k) for k = n. (For example, if n = 5 and π takes values at 1, 2, 3, 4, 5 of 2, 1, 4, 5, 3, then π has a local maximum of 2 at k = 1, and a local maximum of 5 at k = 4.) What is the average number of local maxima of a permutation of S, averaging over all permutations of S? A5 Let n be a positive odd integer and let θ be a real number such that θ/π is irrational. Set a k = tan(θ + kπ/n), k = 1, 2, . . . , n. Prove that a 1 + a 2 + ···+ a n a 1 a 2 ···a n is an integer, and determine its value. A6 Four points are chosen uniformly and independently at random in the interior of a given circle. Find the proba- bility that they are the vertices of a convex quadrilateral. B1 Show that the curve x 3 + 3xy + y 3 = 1 contains only one set of three distinct points, A, B, and C, which are vertices of an equilateral triangle, and find its area. B2 Prove that, for every set X = {x 1 , x 2 , . . . , x n } of n real numbers, there exists a non-empty subset S of X and an integer m such that      m +  s∈S s      ≤ 1 n + 1 . B3 Let S be a finite set of points in the plane. A linear partition of S is an unordered pair {A, B} of subsets of S such that A ∪ B = S, A ∩ B = ∅, and A and B lie on opposite sides of some straight line disjoint from S (A or B may be empty). Let L S be the number of linear partitions of S. For each positive integer n, find the maximum of L S over all sets S of n points. B4 Let Z denote the set of points in R n whose coordinates are 0 or 1. (Thus Z has 2 n elements, which are the vertices of a unit hypercube in R n .) Given a vector sub- space V of R n , let Z(V ) denote the number of members of Z that lie in V . Let k be given, 0 ≤ k ≤ n. Find the maximum, over all vector subspaces V ⊆ R n of dimension k, of the number of points in V ∩ Z.) [Ed- itorial note: the proposers probably intended to write Z(V ) for “the number of points in V ∩ Z”, but this changes nothing.] B5 For each continuous function f : [0, 1] → R, let I(f) =  1 0 x 2 f(x) dx and J(x) =  1 0 x (f(x)) 2 dx. Find the maximum value of I(f) −J(f) over all such functions f. B6 Let k be an integer greater than 1. Suppose a 0 > 0, and define a n+1 = a n + 1 k √ a n for n > 0. Evaluate lim n→∞ a k+1 n n k . This is trial version www.adultpdf.com The 68th William Lowell Putnam Mathematical Competition Saturday, December 1, 2007 A1 Find all values of α for which the curves y = αx 2 + αx + 1 24 and x = αy 2 + αy + 1 24 are tangent to each other. A2 Find the least possible area of a convex set in the plane that intersects both branches of the hyperbola xy = 1 and both branches of the hyperbola xy = −1. (A set S in the plane is called convex if for any two points in S the line segment connecting them is contained in S.) A3 Let k be a positive integer. Suppose that the integers 1, 2, 3, . . . , 3k + 1 are written down in random order. What is the probability that at no time during this pro- cess, the sum of the integers that have been written up to that time is a positive integer divisible by 3? Your answer should be in closed form, but may include fac- torials. A4 A repunit is a positive integer whose digits in base 10 are all ones. Find all polynomials f with real coeffi- cients such that if n is a repunit, then so is f (n). A5 Suppose that a finite group has exactly n elements of order p, where p is a prime. Prove that either n = 0 or p divides n + 1. A6 A triangulation T of a polygon P is a finite collection of triangles whose union is P , and such that the inter- section of any two triangles is either empty, or a shared vertex, or a shared side. Moreover, each side is a side of exactly one triangle in T . Say that T is admissible if every internal vertex is shared by 6 or more triangles. For example, [figure omitted.] Prove that there is an integer M n , depending only on n, such that any admis- sible triangulation of a polygon P with n sides has at most M n triangles. B1 Let f be a polynomial with positive integer coefficients. Prove that if n is a positive integer, then f (n) divides f(f (n) + 1) if and only if n = 1. [Editor’s note: one must assume f is nonconstant.] B2 Suppose that f : [0, 1] → R has a continuous derivative and that  1 0 f(x) dx = 0. Prove that for every α ∈ (0, 1),      α 0 f(x) dx     ≤ 1 8 max 0≤x≤1 |f ′ (x)|. B3 Let x 0 = 1 and for n ≥ 0, let x n+1 = 3x n + ⌊x n √ 5⌋. In particular, x 1 = 5, x 2 = 26, x 3 = 136, x 4 = 712. Find a closed-form expression for x 2007 . (⌊a⌋ means the largest integer ≤ a.) B4 Let n be a positive integer. Find the number of pairs P, Q of polynomials with real coefficients such that (P (X)) 2 + (Q(X)) 2 = X 2n + 1 and deg P > deg Q. B5 Let k be a positive integer. Prove that there exist poly- nomials P 0 (n), P 1 (n), . . . , P k−1 (n) (which may de- pend on k) such that for any integer n,  n k  k = P 0 (n) + P 1 (n)  n k  + ···+ P k−1 (n)  n k  k−1 . (⌊a⌋ means the largest integer ≤ a.) B6 For each positive integer n, let f (n) be the number of ways to make n! cents using an unordered collection of coins, each worth k! cents for some k, 1 ≤ k ≤ n. Prove that for some constant C, independent of n, n n 2 /2−Cn e −n 2 /4 ≤ f(n) ≤ n n 2 /2+Cn e −n 2 /4 . This is trial version www.adultpdf.com The 69th William Lowell Putnam Mathematical Competition Saturday, December 6, 2008 A1 Let f : R 2 → R be a function such that f(x, y) + f(y, z) + f(z, x) = 0 for all real numbers x, y, and z. Prove that there exists a function g : R → R such that f(x, y) = g(x) − g(y ) for all real numbers x and y. A2 Alan and Barbara play a game in which they take turns filling entries of an initially empty 2008 × 2008 array. Alan plays first. At each turn, a player chooses a real number and places it in a vacant entry. The game ends when all the entries are filled. Alan wins if the determi- nant of the resulting matrix is nonzero; Barbara wins if it is zero. Which player has a winning strategy? A3 Start with a finite sequence a 1 , a 2 , . . . , a n of positive integers. If possible, choose two indices j < k such that a j does not divide a k , and replace a j and a k by gcd(a j , a k ) and lcm(a j , a k ), respectively. Prove that if this process is repeated, it must eventually stop and the final sequence does not depend on the choices made. (Note: gcd means greatest common divisor and lcm means least common multiple.) A4 Define f : R → R by f(x) =  x if x ≤ e xf(ln x) if x > e. Does  ∞ n=1 1 f (n) converge? A5 Let n ≥ 3 be an integer. Let f(x) and g(x) be polynomials with real coefficients such that the points (f(1), g(1)), (f(2), g(2)), . . . , (f(n), g(n)) in R 2 are the vertices of a regular n-gon in counterclockwise or- der. Prove that at least one of f(x) and g(x) has degree greater than or equal to n − 1. A6 Prove that there exists a constant c > 0 such that in ev- ery nontrivial finite group G there exists a sequence of length at most c ln |G| with the property that each el- ement of G equals the product of some subsequence. (The elements of G in the sequence are not required to be distinct. A subsequence of a sequence is obtained by selecting some of the terms, not necessarily consec- utive, without reordering them; for example, 4, 4, 2 is a subsequence of 2, 4, 6, 4, 2, but 2, 2, 4 is not.) B1 What is the maximum numberof rational points that can lie on a circle in R 2 whose center is not a rational point? (A rational point is a point both of whose coordinates are rational numbers.) B2 Let F 0 (x) = ln x. For n ≥ 0 and x > 0, let F n+1 (x) =  x 0 F n (t) dt. Evaluate lim n→∞ n!F n (1) ln n . B3 What is the largest possible radius of a circle contained in a 4-dimensional hypercube of side length 1? B4 Let p be a prime number. Let h(x) be a polynomial with integer coefficients such that h(0), h(1), . . . , h(p 2 − 1) are distinct modulo p 2 . Show that h(0), h(1), . . . , h(p 3 − 1) are distinct modulo p 3 . B5 Find all continuously differentiable functions f : R → R such that for every rational number q, the number f(q) is rational and has the same denominator as q. (The denominator of a rational number q is the unique positive integer b such that q = a/b for some integer a with gcd(a, b) = 1.) (Note: gcd means greatest com- mon divisor.) B6 Let n and k be positive integers. Say that a permutation σ of {1, 2, . . ., n} is k-limited if |σ(i ) − i| ≤ k for all i. Prove that the number of k-limited permutations of {1, 2, . . ., n} is odd if and only if n ≡ 0 or 1 (mod 2k + 1). This is trial version www.adultpdf.com Solutions to the 62nd William Lowell Putnam Mathematical Competition Saturday, December 1, 2001 Manjul Bhargava, Kiran Kedlaya, and Lenny Ng A–1 The hypothesis implies for all (by replacing by ), and hence for all (using ). A–2 Let denote the desired probability. Then , and, for , The recurrence yields , , and by a simple induction, one then checks that for general one has . Note: Richard Stanley points out the following nonin- ductive argument. Put ; then the coefficient of in is the probability of getting exactly heads. Thus the desired number is , and both values of can be com- puted directly: , and A–3 By the quadratic formula, if , then , and hence the four roots of are given by . If factors into two nonconstant polynomials over the integers, then some subset of consisting of one or two elements form the roots of a polynomial with integer coefficients. First suppose this subset has a single element, say ; this element must be a rational number. Then is an integer, so is twice a perfect square, say . But then is only rational if , i.e., if . Next, suppose that the subset contains two elements; then we can take it to be one of , or . In all cases, the sum and the product of the elements of the subset must be a ratio- nal number. In the first case, this means , so is a perfect square. In the second case, we have , contradiction. In the third case, we have , or , which means that is twice a perfect square. We conclude that factors into two nonconstant polynomials over the integers if and only if is either a square or twice a square. Note: a more sophisticated interpretation of this argu- ment can be given using Galois theory. Namely, if is neither a square nor twice a square, then the number fields and are distinct quadratic fields, so their compositum is a number field of degree 4, whose Galois group acts transitively on . Thus is irreducible. A–4 Choose so that , and let denote the area of triangle . Then since the tri- angles have the same altitude and base. Also , and (e.g., by the law of sines). Adding this all up yields or . Similarly . Let be the function given by ; then . However, is strictly decreasing in , so is increas- ing and is decreasing. Thus there is at most one such that ; in fact, since the equa- tion has a positive root , we must have . We now compute , , analogously , and . Note: the key relation can also be de- rived by computing using homogeneous coordinates or vectors. A–5 Suppose . Notice that is a multiple of ; thus divides . Since is divisible by 3, we must have , otherwise one of and is a mul- tiple of 3 and the other is not, so their difference cannot be divisible by 3. Now , so we must have , which forces to be even, and in particular at least 2. If is even, then . Since is even, . Since This is trial version www.adultpdf.com , this is impossible. Thus is odd, and so must divide . Moreover, , so . Of the divisors of , those congruent to 1 mod 3 are precisely those not divisible by 11 (since 7 and 13 are both congruent to 1 mod 3). Thus divides . Now is only possible if divides . We cannot have , since for any . Thus the only possibility is . One eas- ily checks that is a solution; all that remains is to check that no other works. In fact, if , then . But since is even, contradiction. Thus is the unique solution. Note: once one has that is even, one can use that is divisible by to rule out cases. A–6 The answer is yes. Consider the arc of the parabola inside the circle , where we initially assume that . This intersects the circle in three points, and . We claim that for sufficiently large, the length of the parabolic arc between and is greater than , which im- plies the desired result by symmetry. We express us- ing the usual formula for arclength: where we have artificially introduced into the inte- grand in the last step. Now, for , since diverges, so does . Hence, for sufficiently large , we have , and hence . Note: a numerical computation shows that one must take to obtain , and that the maximum value of is about , achieved for . B–1 Let (resp. ) denote the set of red (resp. black) squares in such a coloring, and for , let denote the number written in square , where . Then it is clear that the value of depends only on the row of , while the value of depends only on the column of . Since every row contains exactly elements of and elements of , Similarly, because every column contains exactly elements of and elements of , It follows that as desired. Note: Richard Stanley points out a theorem of Ryser (see Ryser, Combinatorial Mathematics, Theorem 3.1) that can also be applied. Namely, if and are matrices with the same row and column sums, then there is a sequence of operations on matrices of the form or vice versa, which transforms into . If we iden- tify 0 and 1 with red and black, then the given color- ing and the checkerboard coloring both satisfy the sum condition. Since the desired result is clearly true for the checkerboard coloring, and performing the matrix op- erations does not affect this, the desired result follows in general. B–2 By adding and subtracting the two given equations, we obtain the equivalent pair of equations Multiplying the former by and the latter by , then adding and subtracting the two resulting equations, we obtain another pair of equations equivalent to the given ones, It follows that and is the unique solution satisfying the given equations. B–3 Since and , we have that if and only if 2 This is trial version www.adultpdf.com [...]... acute Then the remaining points all lie in the arc from the antipode of Q1 to Q1 , but Q2 cannot lie in the arc, and the remaining points cannot all lie in the arc from the antipode of Q1 to the antipode of Q2 Given the choice of Q1 , Q2 , let x be the measure of the counterclockwise arc from Q1 to Q2 ; then the probability that the other points fall into position is 2n+2 xn2 if x 1/2 and 0 otherwise... components by 1/4 1/8 = 1/8 If the i-th square does abut the left edge of the board, the situation is even simpler: if the i-th square differs in color from the square above it, one component is added, otherwise the number does not change Hence adding the i-th square increases the expected number of components by 1/2; likewise if the i-th square abuts the top edge of the board Thus the expected number of components... one of the points Q1 , the angle at Q1 is not acute but the following angle is, and then multiply by n Imagine picking the points by rst choosing Q1 , then picking n 1 pairs of antipodal points and then picking one member of each pair Let R2 , , Rn be the points of the pairs which lie in the semicircle, taken in order away from Q1 , and let S2 , , Sn be the antipodes of these Then to get the desired... coordinates, the dot product is at least a2 b Hence an a2 b, whence n ab as desired Second solution: (by Richard Stanley) Suppose without loss of generality that the a ì b submatrix occupies the rst a rows and the rst b columns Let M be the submatrix occupying the rst a rows and the last n b columns Then the hypothesis implies that the matrix M M T has n bs on the main diagonal and bs elsewhere Hence the. .. 1/8, the i-th square is the same in color as the adjacent squares directly above and to the left of it, but opposite in color from its diagonal neighbor above and to the left In this case, adding the i-th square either removes a component or leaves the number unchanged In all other cases, the number of components remains unchanged upon adding the i-th square Hence adding the i-th square increases the. .. and compare the terms on both sides in which k of the terms are among the ai On the left, one has the product of each kelement subset of {1, , n}; on the right, one has n k/n ã ã ã (b1 bn )(nk)/n , which is prek (a1 ã ã ã an ) n cisely k times the geometric mean of the terms on the left Thus AM-GM shows that the terms under consideration on the left exceed those on the right; adding these inequalities... Elkies) Number the squares of the checkerboard 1, , mn by numbering the rst row from left to right, then the second row, and so on We prove by induction on i that if we just consider the gure formed by the rst i squares, its expected number of monochromatic components is at least i/8 For i = 1, this is clear Suppose the i-th square does not abut the left edge or the top row of the board Then we may... then is r1 r2 |f (x) + f (y)| dx dy E+ ìE Let be an automorphism of the eld of algebraic numbers; then maps each ri to another one, and xes the rational number r1 + r2 If (r1 ) equals one of r1 or r2 , then (r2 ) must equal the other one, and vice versa Thus either xes the set {r1 , r2 } or moves it to {r3 , r4 } But if the latter happened, we would have r1 + r2 = r3 + r4 , contrary to hypothesis... lies below the line y = y0 On the other hand, the points (0, 0), (a, 0), and P divide the boundary of R into three sections The This is trial version www.adultpdf.com 4 length of the section between (0, 0) and P is at least the distance between (0, 0) and P , which is at least y0 ; the length of the section between P and (a, 0) is similarly at least y0 ; and the length of the section between (0, 0) and... multiplicity) in the interval [0, 2), h does also by repeated application of Rolles theorem Since g (e2i ) = 2ie2i h (), g (z 2 ) has at least 2n roots on the unit circle Since g (z 2 ) is equal to z n1 times a polynomial of degree 2n, g (z 2 ) has all roots on the unit circle, as then does g (z) Remarks: The second solution imitates the proof of the Gauss-Lucas theorem: the roots of the derivative of . Write the numbers in the squares of an grid so that the -th row, from left to right, is Color the squares of the grid so that half of the squares in each row and in each column are red and the other half. pre- cisely  n k  times the geometric mean of the terms on the left. Thus AM-GM shows that the terms under con- sideration on the left exceed those on the right; adding these inequalities over all k yields the desired. increasing order. Show that if the sequence b i+1 − b i is unbounded, then lim x→∞ N(x)/x = 0. This is trial version www.adultpdf.com The 66th William Lowell Putnam Mathematical Competition Saturday,