OFFSET
0,2
FORMULA
G.f.: A(x) = Product_{n>=1} [ 1 - (n+1)(n+2)(n+3)/3!*x^n + 3n(n+2)(n+3)/3!*x^(n+1) - 3n(n+1)(n+3)/3!*x^(n+2) + n(n+1)(n+2)/3!*x^(n+3) ].
EXAMPLE
G.f.: A(x) = (1-4x+6x^2-4x^3+x^4)*(1-10x^2+20x^3-15x^4+4x^5)*(1-20x^3+45x^4-36x^5+10x^6)*(1-35x^4+84x^5-70x^6+20x^7)*...
Terms are divisible by 4 except at positions given by:
a(n) == 1 (mod 4) at n = 16*[0, 2, 5, 15, 22, 26, 40,...];
a(n) == -1 (mod 4) at n = 16*[1, 7, 12, 35, 51, 57,...];
a(n) == 2 (mod 4) at n = 8*[1,3,5,6,8,9,13,15,16,19,..,A129359(k),..].
PROG
(PARI) {a(n)=if(n==0, 1, polcoeff(prod(k=1, n, (1-x)^4*sum(j=1, k, binomial(j+2, 3)*x^(j-1)) +x*O(x^n)), n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Paul D. Hanna, Apr 11 2007
STATUS
approved