OFFSET
1,1
EXAMPLE
If n=111, 2n=222, 2n//n = 222111 = 666*667/2, a triangular number.
MATHEMATICA
TriangularQ[n_] := IntegerQ[Sqrt[1 + 8*n]]; t = {}; Do[s = FromDigits[Join[IntegerDigits[2*n], IntegerDigits[n]]]; If[TriangularQ[s], AppendTo[t, s]], {n, 100000}]; t (* T. D. Noe, Jun 18 2013 *)
PROG
(PARI)
concatint(a, b)=eval(concat(Str(a), Str(b)))
istriang(x)=issquare(8*x+1)
{for(n=1, 10^5, a=concatint(2*n, n); if(istriang(a), print(a)))}
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Antonio Roldán, Jun 18 2013
STATUS
approved