S000819


Numbers that are not the sum of 3 generalized 20-gonal numbers.

4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 36, 39, 42, 43, 44, 45, 46, 47, 48, 49, 50, 55, 56, 61, 62, 63, 64, 65, 66, 67, 68, 71, 76, 79, 80, 81, 82, 83, 84, 85, 87, 88, 90, 93, 95, 96, 98, 99, 100, 101, 102, 103

1

S000819

This appears to be an infinite series.

T. D. Noe, Plot of 1000 terms

T. D. Noe, Table of 1000 terms

Eric W. Weisstein, MathWorld: Polygonal Number

(Mma) f = 20; nn = 100; t = Table[k ((f - 2) k - (f - 4))/2, {k, -nn, nn}]; t2 = Select[Union[Flatten[Table[a + b + c, {a, t}, {b, t}, {c, t}]]], # <= t[[-1]] &]; Complement[Range[t[[-1]]], t2]

Cf. A118282, S000806-S000818.

nonn

T. D. Noe, Dec 28 2015

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