Palindromic triangular numbers in base 4.
1, 3, 22, 33, 111, 313, 1221, 2332, 11011, 21312, 22122, 333333, 1102011, 31111113, 3223223223, 3333003333, 11201310211, 31203330213, 1010111110101, 2022210122202, 2100120210012, 3010113110103, 33100233200133, 100112222211001, 133131111131331
1
See S000346 for the n that produce these triangular numbers.
T. D. Noe, Plot of 47 terms
T. D. Noe, Table of 47 terms
Eric W. Weisstein, MathWorld: Triangular Number
(Mma) ff[base_, len_] := Module[{n = 0, t = {}, tri, d}, While[Length[t] < len, n++; tri = n*(n + 1)/2; d = IntegerDigits[tri, base]; If[d == Reverse[d], AppendTo[t, n]]]; t]; t4 = ff[4, 20]; Table[FromDigits[IntegerDigits[n*(n + 1)/2, 4]], {n, t4}]
Cf. S000346.
nonn,base,hard
T. D. Noe, Nov 19 2014