OFFSET
1,2
COMMENTS
The bivariate polynomials P(n,x,y) := (x^n + y^n)*(x^(3*n) - y^(3*n)) form a sequence of divisibility polynomials in the polynomial ring Z[x,y]; that is, if n divides m then P(n,x,y) divides P(m,x,y) in Z[x,y] (see the Bala link). Here we consider the integer sequence coming from the normalized polynomials P(n,x,y)/P(1,x,y) when x = 3 and y = 2. Other cases include A238538(x = 2, y = 1), A238539(x = 2, y = -1) and A238540(x = 3, y = 1). See also A238536, A238537 and A215466.
LINKS
Wikipedia, Divisibility sequence
Index entries for linear recurrences with constant coefficients, signature (175,-10158,226800,-1679616).
FORMULA
a(n) = (1/95)*(3^n + 2^n)*(3^(3*n) - 2^(3*n)).
a(n) = (1/95)*(9^n - 4^n)*(27^n - 8^n)/(3^n - 2^n).
O.g.f.: x*(1 - 84*x + 1296*x^2)/((1 - 16*x)*(1 - 24*x)*(1 - 54*x)*(1 - 81*x)).
Recurrence equation: a(n) = 175*a(n-1) - 10158*a(n-2) + 226800*a(n-4) - 1679616*a(n-4).
MAPLE
seq(1/95*(3^n + 2^n)*(3^(3*n) - 2^(2*n)), n = 1..20);
MATHEMATICA
LinearRecurrence[{175, -10158, 226800, -1679616}, {1, 91, 7063, 538447}, 20] (* Harvey P. Dale, Apr 12 2018 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Peter Bala, Mar 01 2014
STATUS
approved