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A226772 Triangular numbers obtained as the concatenation of n and 2n. 4
36, 1326, 2346, 3570, 125250, 223446, 12502500, 22234446, 1250025000, 2066441328, 2222344446, 2383847676, 3673573470, 125000250000, 222223444446, 5794481158896, 12500002500000, 12857132571426, 22222234444446, 49293309858660, 804878916097578, 933618918672378, 971908519438170 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..23.

EXAMPLE

If n=23, 2n=46, n//2n = 2346 = 68*69/2, a triangular number.

MATHEMATICA

TriangularQ[n_] := IntegerQ[Sqrt[1 + 8*n]]; t = {}; Do[s = FromDigits[Join[IntegerDigits[n], IntegerDigits[2*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(n, 2*n); if(istriang(a), print(a)))}

CROSSREFS

Cf. A003098, A068899, A226742, A226788, A226789.

Sequence in context: A209042 A283729 A203333 * A158739 A218518 A099366

Adjacent sequences:  A226769 A226770 A226771 * A226773 A226774 A226775

KEYWORD

nonn,base

AUTHOR

Antonio Roldán, Jun 18 2013

STATUS

approved

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Last modified September 21 19:29 EDT 2018. Contains 315260 sequences. (Running on oeis4.)