A077400 Triangular numbers that are 7 times triangular numbers.
0, 21, 105, 5460, 26796, 1386945, 6806205, 352278696, 1728749400, 89477401965, 439095541521, 22726907820540, 111528538797060, 5772545109015321, 28327809758911845, 1466203730782071120, 7195152150224811696, 372409975073537049285, 1827540318347343259065
Offset: 0
Links
- Colin Barker, Table of n, a(n) for n = 0..831
- Vladimir Pletser, Recurrent Relations for Multiple of Triangular Numbers being Triangular Numbers, arXiv:2101.00998 [math.NT], 2021.
- Vladimir Pletser, Triangular Numbers Multiple of Triangular Numbers and Solutions of Pell Equations, arXiv:2102.13494 [math.NT], 2021.
- Vladimir Pletser, Using Pell equation solutions to find all triangular numbers multiple of other triangular numbers, 2022.
- Index entries for linear recurrences with constant coefficients, signature (1,254,-254,-1,1).
Programs
-
Magma
Q:=Rationals(); R
:=PowerSeriesRing(Q,30); [0] cat Coefficients(R!(-21*x*(x^2+4*x+1) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)))); // G. C. Greubel, Jan 18 2018 -
Maple
f := gfun:-rectoproc({a(-2) = 21, a(-1) = 0, a(0) = 0, a(1) = 21, a(n) = 254*a(n-2)-a(n-4)+126}, a(n), remember): map(f, [`$`(0 .. 1000)])[] # Vladimir Pletser, Feb 21 2021 -
Mathematica
LinearRecurrence[{1,254,-254,-1,1},{0,21,105,5460,26796},20] (* Harvey P. Dale, Oct 28 2013 *) -
PARI
concat(0, Vec(-21*x*(x^2+4*x+1)/((x-1)*(x^2-16*x+1)*(x^2+16*x+1)) + O(x^100))) \\ Colin Barker, May 15 2015
Formula
Let b(n) = A077399(n) then a(n) = 7*b(n).
G.f.: -21*x*(x^2+4*x+1) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)). - Colin Barker, Jul 02 2013
From _Vladimir Pletser, Feb 21 2021: (Start)
a(n) = 254*a(n - 2) - a (n - 4) + 126.
a(n) = a(n - 1) + 254*(a(n - 2) - a(n - 3)) - (a (n - 4) - a(n - 5)). (End)