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A245716
Least number k > 0 such that n + k! and n - k! are both prime, or 0 if no such k exists.
3
0, 0, 0, 1, 2, 1, 0, 0, 2, 0, 3, 1, 3, 0, 2, 0, 3, 1, 0, 0, 2, 0, 3, 0, 3, 0, 0, 0, 4, 1, 0, 0, 0, 0, 3, 0, 3, 0, 2, 0, 0, 1, 4, 0, 2, 0, 3, 0, 0, 0, 0, 0, 3, 0, 4, 0, 0, 0, 0, 1, 0, 0, 0, 0, 3, 0, 3, 0, 2, 0, 0, 1, 3, 0, 0, 0, 3, 0, 0, 0, 2, 0, 4, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 2, 0, 0, 1, 3, 0, 2, 0, 3, 1, 0, 0, 2, 0, 4, 0, 0, 0, 0, 0, 0, 0
OFFSET
1,5
COMMENTS
For a(n) > 0, a(n)! < n for all n. Thus a(n) = 0 is definite.
LINKS
EXAMPLE
13 + 1! and 13 - 1! are not both prime.
13 + 2! and 13 - 2! are not both prime.
13 + 3! and 13 - 3! are both prime (19 and 7). Thus a(13) = 3.
PROG
(PARI)
a(n)=for(k=1, n, if(ispseudoprime(n-k!)&&ispseudoprime(n+k!), return(k)))
vector(150, n, a(n))
CROSSREFS
Sequence in context: A035225 A298931 A035219 * A241425 A352560 A106347
KEYWORD
nonn
AUTHOR
Derek Orr, Jul 30 2014
STATUS
approved