136 Solving Initial Condition Problems with llie linpliic.e 1kaiistr)rin (7.53) ,4t tlie niiclrllr srnioiliirig iiode (compare (2.15) for N = 2) IEiflr\rc\rrtiiLting (7.53);txid substituting in (7.54) yicAlt1s (7.55)