A264738 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is triangular (m*(m+1)/2).

1, 5, 3, 181, 88028, 21235051740, 567915201450407862150, 344114043575504570099838908555592732193828, 2517587700386740093077985318903210991887701682095104296188806385635425261250603998340

Offset

1,2

Links

Jon E. Schoenfield, Table of n, a(n) for n = 1..12

Wikipedia, Triangular_number

Prog

(PARI) istriangular(n)=ispolygonal(n, 3)
first(m)=my(v=vector(m), s="1"); v[1]=1; for(i=2, m, n=1; while(!istriangular(eval(concat(s, Str(n)))), n++); v[i]=n; s=concat(s, Str(n))); v

Crossrefs

Cf. A000217, A069606.

Sequence in context: A267512 A230389 A048885 * A002208 A100653 A105318

Adjacent sequences: A264735 A264736 A264737 * A264739 A264740 A264741

Keyword

nonn,base

Author

Anders Hellström, Nov 22 2015

Extensions

a(6)-a(9) from Jon E. Schoenfield, Nov 22 2015

Status

approved