OFFSET
0,1
FORMULA
a(n) = minimal value of k > 0 such that n^2 + k^2 is semiprime.
EXAMPLE
a(0) = 2 because 0^2 + 1^2 = 1 is not semiprime, but 0^2 + 2^2 = 4 = 2^2 is.
a(1) = 3 because 1^2 + 1^2 and 1^2 + 2^2 are not semiprime, but 1^2 + 3^2 = 10 = 2 * 5 is semiprime.
a(81) = 14 because 81^2 + 14^2 = 6757 = 29 * 233 and for no smaller k>0 is 81^2 + k^2 a semiprime.
a(100) = 1 because 100^2 + 1^2 = 10001 = 73 * 137.
MATHEMATICA
k2sp[n_]:=Module[{k=1}, While[PrimeOmega[n^2+k^2]!=2, k++]; k]; Array[ k2sp, 110, 0] (* Harvey P. Dale, Oct 30 2016 *)
PROG
(PARI) A109197(n)={local(r); r=1; while(bigomega(n^2+r^2)<>2, r=r+1); r} \\ Michael B. Porter, May 13 2010
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Jun 21 2005
STATUS
approved