OFFSET
1,5
COMMENTS
If k > n, n - k! is surely negative and, therefore, not prime.
a(n) < A020639(n). - Robert Israel, Aug 10 2014
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
a:= proc(n)
local k;
for k from min(numtheory:-factorset(n))-1 to 1 by -1 do
if n > k! and isprime(n+k!) and isprime(n-k!) then return(k) fi
od:
0
end proc:
a(1):= 0:
seq(a(n), n=1..100); # Robert Israel, Aug 10 2014
MATHEMATICA
a[n_] := Module[{k}, For[k = FactorInteger[n][[1, 1]], k >= 1, k--, If[n > k! && PrimeQ[n + k!] && PrimeQ[n - k!], Return[k]]]; 0];
a[1] = 0;
Array[a, 100] (* Jean-François Alcover, Jul 27 2020, after Maple *)
PROG
(PARI)
a(n)=forstep(k=n, 1, -1, if(ispseudoprime(n+k!)&&ispseudoprime(n-k!), return(k)))
n=1; while(n<150, print1(a(n), ", "); n++)
CROSSREFS
KEYWORD
nonn
AUTHOR
Derek Orr, Aug 08 2014
STATUS
approved