OFFSET
1,5
EXAMPLE
a(11) = 3 because 11^2 - 6^2 = 85, 11^2 - 8^2 = 57 and 11^2 - 10^2 = 21 are the only three semiprimes of the form 11^2 - k^2 with 0 < k < 11.
MATHEMATICA
a[n_] := Sum[Boole[ PrimeOmega[n^2 - k^2] == 2], {k, n-1}]; Array[a, 96] (* Giovanni Resta, Jul 13 2018 *)
PROG
(PARI) a(n) = sum(k=1, n-1, bigomega(n^2-k^2)==2); \\ Michel Marcus, Jul 12 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Arnauld Chevallier, Jul 12 2018
STATUS
approved