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 A226742 Triangular numbers obtained as the concatenation of 2n and n. 3
 21, 105, 2211, 9045, 222111, 306153, 742371, 890445, 1050525, 22221111, 88904445, 107905395, 173808690, 2222211111, 8889044445, 12141260706, 15754278771, 222222111111, 888890444445, 22222221111111, 36734701836735, 65306123265306 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS 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 Cf. A003098, A068899, A226772, A226788, A226789. Sequence in context: A279763 A068986 A219631 * A077400 A041854 A219820 Adjacent sequences:  A226739 A226740 A226741 * A226743 A226744 A226745 KEYWORD nonn,base AUTHOR Antonio Roldán, Jun 18 2013 STATUS approved

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Last modified February 24 01:16 EST 2020. Contains 332195 sequences. (Running on oeis4.)