OFFSET
1,1
COMMENTS
Solution to a*b = (b*(b-1) - a*(a+1))/2 in A000027 with a,b >= 2.
LINKS
Colin Barker, Table of n, a(n) for n = 1..650
Index entries for linear recurrences with constant coefficients, signature (41,-246,246,-41,1).
FORMULA
From Colin Barker, Mar 31 2018: (Start)
G.f.: 2*x*(2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)).
a(n) = 41*a(n-1) - 246*a(n-2) + 246*a(n-3) - 41*a(n-4) + a(n-5) for n>5.
(End)
a(n) = (1 - 8*U(n, 3) + 7*U(n, 17) + U(n-1, 17)) / 16 where U(n, x) is the Chebyshev polynomial of the second kind. - Michael Somos, Jul 18 2018
EXAMPLE
A053141(2) = 14 and A001109(3) = 35, then 14*35 = 15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30+31+32+33+34 = 490, is a term.
G.f. = 12*x + 490*x^2 + 17136*x^3 + 584988*x^4 + 19889100*x^5 + ... - Michael Somos, Jul 18 2018
MATHEMATICA
lst={}; k=1; j=0; s=1; Do[a=6*k-j; p=2*s*a; s=s+a; AppendTo[lst, p]; j=k; k=a, {n, 1, 16}]; lst
LinearRecurrence[{41, -246, 246, -41, 1}, {12, 490, 17136, 584988, 19889100}, 30] (* G. C. Greubel, Jul 15 2018 *)
a[ n_] := (1 - 8 ChebyshevU[n, 3] + 7 ChebyshevU[n, 17] + ChebyshevU[n - 1, 17]) / 16; (* Michael Somos, Jul 18 2018 *)
PROG
(PARI) Vec(2*x*(2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + O(x^20)) \\ Colin Barker, Mar 31 2018
(PARI) {a(n) = if( n>0, polcoeff( 2*x * (2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + x * O(x^n), n), n=-n; polcoeff( -2*x^2 * (1-2*x)*(1+3*x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + x * O(x^n), n))}; /* Michael Somos, Jul 18 2018 */
(Magma) m:=25; R<x>:=PowerSeriesRing(Integers(), m); Coefficients(R!(2*x*(2-x)*(3+x)/((1-x)*(1-34*x+x^2)*(1-6*x+x^2)))); // G. C. Greubel, Jul 15 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Manuel Valdivia, Apr 03 2010
EXTENSIONS
Definition simplified by N. J. A. Sloane, Apr 05 2010
STATUS
approved