OFFSET
0,1
COMMENTS
EXAMPLE
4*11!-1 = 159667199 that is prime of the form 4*k+3, hence a(11) = 159667199.
4*13!-1 = 24908083199 = 47 * 2963 * 178859, these 3 primes factors are all of the form 4*k+3, the smallest one is 47 hence a(13) = 47.
4*14!-1 = 348713164799 = 61 * 1901 * 3007159, only 3007159 is a prime of the form 4*k+3, hence a(14) = 3007159.
MATHEMATICA
a[n_] := Min[Select[First /@ FactorInteger[4*n! - 1], Mod[#, 4] == 3 &]]; Array[a, 30, 0] (* Amiram Eldar, Apr 10 2020 *)
PROG
(PARI) a(n) = {my(f=factor(4*n!-1)[, 1]); for(i=1, #f, if(f[i]%4==3, return(f[i]))); } \\ Jinyuan Wang, Apr 10 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Bernard Schott, Apr 10 2020
EXTENSIONS
a(23) corrected by and more terms from Jinyuan Wang, Apr 10 2020
STATUS
approved