Numbers that are not the sum of 3 generalized 18-gonal numbers.
4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 21, 22, 23, 24, 25, 26, 27, 28, 29, 32, 35, 38, 39, 40, 41, 42, 43, 44, 49, 50, 55, 56, 57, 58, 59, 60, 63, 68, 71, 72, 73, 74, 75, 77, 78, 80, 83, 85, 86, 88, 89, 90, 91, 96, 99, 104, 105, 106, 113, 114, 121, 122, 124, 125
1
It appears that 212048 is the last number not representable as the sum of 3 generalized 18-gonal numbers.
T. D. Noe, Plot of 743 terms
T. D. Noe, Table of 743 terms
Eric W. Weisstein, MathWorld: Polygonal Number
(Mma) f = 18; nn = 200; 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]
nonn,fini
T. D. Noe, Dec 28 2015