The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A264738 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is triangular (m*(m+1)/2). 9

%I

%S 1,5,3,181,88028,21235051740,567915201450407862150,

%T 344114043575504570099838908555592732193828,

%U 2517587700386740093077985318903210991887701682095104296188806385635425261250603998340

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

%H Jon E. Schoenfield, <a href="/A264738/b264738.txt">Table of n, a(n) for n = 1..12</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Triangular_number">Triangular_number</a>

%o (PARI) istriangular(n)=ispolygonal(n,3)

%o 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

%Y Cf. A000217, A069606.

%K nonn,base

%O 1,2

%A _Anders HellstrÃ¶m_, Nov 22 2015

%E a(6)-a(9) from _Jon E. Schoenfield_, Nov 22 2015

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 26 21:46 EDT 2020. Contains 337377 sequences. (Running on oeis4.)