Palindromic triangular numbers in base 8.
1, 3, 6, 44, 55, 151, 424, 454, 505, 626, 4774, 7777, 13731, 21112, 27472, 37073, 40704, 62426, 167761, 516615, 725527, 2773772, 3535353, 4053504, 4733374, 6063606, 6244426, 6427246, 24355342, 42322324, 51355315, 70000007, 102121201, 120414021, 137000731
1
See S000350 for the n that produce these triangular numbers.
T. D. Noe, Plot of 176 terms
T. D. Noe, Table of 176 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]; t8 = ff[8, 40]; Table[FromDigits[IntegerDigits[n*(n + 1)/2, 8]], {n, t8}]
Cf. S000350.
nonn,base,hard
T. D. Noe, Nov 19 2014