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A248743
Smallest prime Q such that there is a prime R such that Q*(R+1)+1 or Q*(R+1)-1 or Q*(R-1)+1 or Q*(R-1)-1 is prime(n), or 0 if no such prime Q exists.
1
3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 2, 3, 2, 2, 2, 2, 2, 2, 5, 2, 2, 2, 3, 3, 2, 2, 2, 2, 5, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 2, 2, 2, 3, 2, 2, 2, 47, 3, 3, 2, 2, 2, 2, 2, 2, 2, 11, 2, 23, 17, 2, 2, 2, 2, 2, 3
OFFSET
1,1
COMMENTS
The first prime prime(n) with no solution is prime(1300)=10657.
There are 43 terms 0 within the first 10000 primes.
EXAMPLE
2=3*(2-1)-1, so a(1)=3.
3=2*(3-1)-1, so a(2)=2.
5=2*(2+1)-1=2*(3-1)+1, so a(3)=2.
7=2*(2+1)+1=2*(3+1)-1=2*(5-1)-1, so a(4)=2.
PROG
(PARI) a(p) = {if (p == 2, return (3)); forprime (q=2, p, if (!((p-1) % q) && (isprime((p-1)/q+1) || isprime((p-1)/q-1)), return (q)); if (!((p+1) % q) && (isprime((p+1)/q+1) || isprime((p+1)/q-1)), return (q)); ); } \\ to be used with a forprime loop; Michel Marcus, Oct 28 2014
CROSSREFS
Sequence in context: A262626 A104435 A178815 * A085398 A252503 A270003
KEYWORD
nonn
AUTHOR
Pierre CAMI, Oct 13 2014
STATUS
approved