OFFSET
1,2
COMMENTS
All the terms in this sequence are triangular, and hence 0 or 1 (mod 3).
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..10000
EXAMPLE
a(6) = 136 = 16 * (16+1) / 2, that is triangular number. (1+3+6) + (1*3*6) = 28, which is 7th triangular number.
a(15) = 1128 = 47 * (47+1) / 2, that is triangular number. (1+1+2+8) + (1*1*2*8) = 28, which is 7th triangular number.
MAPLE
MATHEMATICA
PROG
(PARI) for(n =0, 500, t = n*(n+1)/2; k = (sumdigits(t)); d = digits (t); p = prod(i = 1, #d, d[i]); s = k+p; if(ispolygonal(s, 3), print1(t, ", ")));
(Magma) [n*(n+1) div 2: n in [0..100] | IsSquare(8*k+1) where k is (&+Intseq(n*(n+1) div 2) + &*Intseq(n*(n+1) div 2))];
CROSSREFS
KEYWORD
nonn,base
AUTHOR
K. D. Bajpai, Aug 09 2015
STATUS
approved