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 A274756 Values of n such that 2*n+1 and 6*n+1 are both triangular numbers. 3
 0, 945, 13167, 35578242, 495540990, 1338951572595, 18649189618605, 50390103447476100, 701843601611053692, 1896381151803363988917, 26413182084381205040235, 71368408216577696911440390, 994033693861758668873164410, 2685878672926303893761783662455 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Intersection of A074377 and A274757. LINKS Colin Barker, Table of n, a(n) for n = 1..400 Index entries for linear recurrences with constant coefficients, signature (1,37634,-37634,-1,1). FORMULA G.f.: 63*x^2*(15+194*x+15*x^2) / ((1-x)*(1-194*x+x^2)*(1+194*x+x^2)). a(n) = a(n-1)+37634*a(n-2)-37634*a(n-3)-a(n-4)+a(n-5). - Wesley Ivan Hurt, Apr 24 2021 EXAMPLE 945 is in the sequence because 2*945+1 = 1891, 6*945+1 = 5671, and 1891 and 5671 are both triangular numbers. PROG (PARI) isok(n) = ispolygonal(2*n+1, 3) && ispolygonal(6*n+1, 3) (PARI) concat(0, Vec(63*x^2*(15+194*x+15*x^2)/((1-x)*(1-194*x+x^2)*(1+194*x+x^2)) + O(x^20))) CROSSREFS Cf. A124174 (2*n+1 and 9*n+1), A274579 (2*n+1 and 5*n+1), A274603 (2*n+1 and 3*n+1), A274680 (2*n+1 and 4*n+1). Sequence in context: A293186 A294027 A127666 * A335053 A290034 A335055 Adjacent sequences:  A274753 A274754 A274755 * A274757 A274758 A274759 KEYWORD nonn,easy AUTHOR Colin Barker, Jul 04 2016 STATUS approved

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Last modified October 22 01:48 EDT 2021. Contains 348160 sequences. (Running on oeis4.)