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 A118278 Conjectured largest number that is not the sum of three n-gonal numbers, or -1 if there is no largest number. 16
 0, -1, 33066, 146858, 273118, -1, 1274522, 2117145, 3613278, -1, 7250758, -1, 12911636, -1, 22655394, 26801303, 25049533, -1, 56922533, 115715602, 81539010, -1, 85105105, -1, 106555658, -1, 233296317, 267370631, 286763923, -1, 358322750 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,3 COMMENTS Extensive calculations show that if a(n) >= 0, then every number greater than a(n) can be represented as the sum of three n-gonal numbers. a(3)=0 because every number can be written as the sum of three triangular numbers. When n is a multiple of 4, there is an infinite set of numbers not representable. For n=14, there appears to be a sparse, but infinite, set of numbers not representable. LINKS Table of n, a(n) for n=3..33. R. K. Guy, Every number is expressible as the sum of how many polygonal numbers?, Amer. Math. Monthly 101 (1994), 169-172. Gordon Pall, Large positive integers are sums of four or five values of a quadratic function, Am. J. Math 54 (1931) 66-78 Eric Weisstein's World of Mathematics, MathWorld: Polygonal Number CROSSREFS Cf. A118279 (number of numbers not representable). Cf. A003679 (not the sum of three pentagonal numbers). Cf. A007536 (not the sum of three hexagonal numbers). Cf. A213523 (not the sum of three heptagonal numbers). Cf. A213524 (not the sum of three octagonal numbers). Cf. A213525 (not the sum of three 9-gonal numbers). Cf. A214419 (not the sum of three 10-gonal numbers). Cf. A214420 (not the sum of three 11-gonal numbers). Cf. A214421 (not the sum of three 12-gonal numbers). Sequence in context: A170789 A237625 A237624 * A118280 A062682 A094889 Adjacent sequences: A118275 A118276 A118277 * A118279 A118280 A118281 KEYWORD sign AUTHOR T. D. Noe, Apr 21 2006 EXTENSIONS a(22)-a(33) from Donovan Johnson, Apr 17 2010 STATUS approved

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Last modified June 1 18:23 EDT 2023. Contains 363076 sequences. (Running on oeis4.)