OFFSET
1,1
COMMENTS
b - a = 17 is the third term in A058529.
LINKS
Index entries for linear recurrences with constant coefficients, signature (1,6,-6,-1,1).
FORMULA
a(1) = 7, a(2) = 28, a(3) = 207, a(4) = 555, a(n) = 6*a(n-2) - a(n-4) + 34.
G.f.: x*(5*x^4 + 7*x^3 - 18*x^2 - 21*x - 7) / ((x-1)*(x^2 - 2*x - 1)*(x^2 + 2*x - 1)). - Colin Barker, Dec 17 2012
EXAMPLE
a(5) = 6*88 - 7 + 34 = 555 and 555^2 + 572^2 = 797^2 and 572 - 555 = 17 and gcd(555, 572, 797) = 1.
MATHEMATICA
CoefficientList[Series[(5x^4 + 7x^3 - 18x^2 - 21x - 7)/((x - 1)(x^2 - 2x - 1)(x^2 + 2x - 1)), {x, 0, 30}], x] (* Wesley Ivan Hurt, Apr 14 2017 *)
LinearRecurrence[{1, 6, -6, -1, 1}, {7, 28, 88, 207, 555}, 30] (* Harvey P. Dale, Jul 31 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Andras Erszegi (erszegi.andras(AT)chello.hu), Mar 19 2006
STATUS
approved