OFFSET
0,3
COMMENTS
36*k^2 + 12*k + 7 = (6*k+1)^2 + 6, which is six more than a square.
FORMULA
a(n) = (-1 +- sqrt(A056909(n) - 6))/6, choosing +- to give an integer result for each n.
EXAMPLE
a(2)=-2 since 36*(-2)^2 + 12*(-2) + 7 = 127, which is prime (as well as being six more than a square).
CROSSREFS
KEYWORD
sign
AUTHOR
Henry Bottomley, Jul 07 2000
STATUS
approved