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A255598
a(n) is the minimal number q>1 such that n(q+1)-1 is prime, or -1 if no such q exists.
0
2, 2, 3, 2, 3, 2, 5, 2, 5, 2, 3, 3, 7, 2, 3, 2, 3, 2, 5, 2, 3, 4, 5, 2, 5, 3, 3, 2, 5, 2, 13, 3, 3, 2, 3, 2, 11, 2, 5, 4, 3, 3, 5, 2, 3, 2, 5, 3, 5, 2, 9, 5, 3, 4, 7, 2, 3, 2, 5, 2, 7, 6, 3, 2, 5, 2, 5, 3, 11, 4, 3, 4, 13, 5, 5, 2, 3, 2, 7, 2, 7, 4, 3, 2, 5, 2, 3, 2, 15, 2, 7, 3, 5, 2, 3, 3, 11
OFFSET
1,1
COMMENTS
a(n) <= A060324(n), and a(n) = A060324(n) iff a(n) is prime.
Among first 10000 terms, in 2453 cases a(n) < A060324(n). First such cases are for n = 22, 40, 51, 54, 62, 70, 72, 82, 89, 100.
Also, among first 10^6 terms, in 324388 cases a(n) < A060324(n).
PROG
(PARI) a(n) = {my(q = 2); while (! isprime(n*(q+1)-1), q ++); q; } \\ Michel Marcus, Feb 27 2015
CROSSREFS
Cf. A060324.
Sequence in context: A054714 A369032 A235922 * A060324 A046216 A105560
KEYWORD
nonn
AUTHOR
Zak Seidov, Feb 27 2015
STATUS
approved