A109908 a(n) = greatest prime of the form k*(n-k)-1, or 0 if no such prime exists.

0, 0, 0, 3, 5, 7, 11, 11, 19, 23, 29, 31, 41, 47, 53, 59, 71, 79, 89, 83, 109, 71, 131, 139, 149, 167, 181, 191, 197, 223, 239, 251, 271, 263, 293, 307, 311, 359, 379, 383, 419, 439, 461, 479, 503, 503, 521, 571, 599, 599, 647, 659, 701, 727, 743, 719, 811, 839

Offset

1,4

Comments

Conjecture: a(n) > 0 for n > 3.

Conjecture verified up to 10^9. - Mauro Fiorentini, Jul 23 2023

Links

Table of n, a(n) for n=1..58.

Prog

(PARI) { a(n)=forstep(k=n\2, 1, -1, if(isprime(k*(n-k)-1), return(k*(n-k)-1))); return(0) } \\ Max Alekseyev, Oct 04 2005

Crossrefs

Cf. A109904, A109905, A026728, A109907, A109909.

Sequence in context: A058024 A215464 A260910 * A372028 A152212 A347465

Adjacent sequences: A109905 A109906 A109907 * A109909 A109910 A109911

Keyword

nonn,easy

Author

Amarnath Murthy, Jul 15 2005

Extensions

More terms from Max Alekseyev, Oct 04 2005

Definition corrected by David Wasserman, Oct 28 2008

Status

approved