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

Author

Anders Hellström, Nov 22 2015

Keywords

• nonn
• base

Links

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

Crossrefs

Cf. A000217, A069606.

Programs

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

Extensions

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