OFFSET
0,2
COMMENTS
a(3n)=a(3n+1)/3=a(3n+2)/21; a(3^k*n)=a(3^k*n+3^(k-1))/a(3^(k-1))=a(3^k*n+2*3^(k-1))/a(2*3^(k-1)), k>0. Divisors of a(9)=4^9-1 include a(0),a(1),a(2),a(3),a(4),a(6),a(7),a(8),a(9).
FORMULA
a(n)=sum{k=0..n, (-1)^(n-k)*L(C(n,k)/3)*4^k} where L(j/p) is the Legendre symbol of j and p.
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Apr 06 2006
STATUS
approved