OFFSET
1,1
COMMENTS
Second term in the smallest non-constant p-term arithmetic progression (AP) of primes beginning with p = prime(n).
For any non-constant AP beginning with a prime p and ending with a prime, the maximum possible length is p, since p+pd is not prime for d > 0. If all the terms are prime, then the common difference d must be a multiple of all primes < p.
Ribenboim says that in 1986 G. Loh [Loeh] discovered a(5) and a(6), and that a(n) should exist for all n, but "in my opinion, this is so difficult that no one will prove [it], and no one will find a counterexample in the near future."
Phil Carmody found a(7) in 2001.
See the crossrefs for more comments, references, and links.
REFERENCES
P. Ribenboim, My Numbers, My Friends, Springer, 2000; p. 67.
P. Ribenboim, The Book of Prime Number Records, 2nd ed., Springer, 1989; p. 225.
LINKS
Phil Carmody, a(7), NMBRTHRY November 2001.
EXAMPLE
Prime(3) = 5 and 5, 11, 17, 23, 29 is the smallest 5-term AP beginning with 5, so a(3) = 11.
PROG
(PARI) a(n)=my(p=prime(n), P=prod(i=1, n-1, prime(i)), d); forprime(q=p+1, , d=q-p; if(d%P, next); for(i=2, p-1, if(!isprime(p+i*d), next(2))); return(q)) \\ Charles R Greathouse IV, Nov 08 2013
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Jonathan Sondow, Nov 08 2013
EXTENSIONS
a(8) found by Wojciech Izykowski, from Jens Kruse Andersen, Jun 30 2014
STATUS
approved