A219789 - OEIS

A219789

Least prime in the form x*y-1 with x > 0, y > 0 and x + y = n > 3.

2, 3, 7, 5, 11, 7, 23, 17, 19, 11, 23, 13, 47, 29, 31, 17, 83, 19, 71, 41, 43, 23, 47, 71, 131, 53, 103, 29, 59, 31, 167, 149, 67, 101, 71, 37, 383, 113, 79, 41, 83, 43, 167, 89, 251, 47, 263, 97, 191, 101, 103, 53, 107, 109, 311, 113, 223, 59, 431, 61, 239, 353

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OFFSET

4,1

COMMENTS

See comments in A091182, A219782 and A219791.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 4..10000

Zhi-Wei Sun, An amazing conjecture on primes, a message to Number Theory List, Nov. 27, 2012.

Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv:1211.1588.

EXAMPLE

a(6)=7 since 6=2+4 with 2*4-1=7 prime.

MATHEMATICA

Do[Do[If[PrimeQ[k(n-k)-1]==True, Print[n, " ", k(n-k)-1]; Goto[aa]], {k, 1, n/2}]; Print[n, " ", counterexample]; Label[aa]; Continue, {n, 4, 100}]

Table[k = 1; While[p = k*(n - k) - 1; k <= n/2 && ! PrimeQ[p], k++]; If[k > n/2, Print["counterexample at n=", n]; p = 0]; p, {n, 4, 100}] (* T. D. Noe, Nov 28 2012 *)

CROSSREFS

Cf. A091182, A219782, A219791.

Sequence in context: A085107 A358242 A241082 * A034694 A050921 A087386

Adjacent sequences: A219786 A219787 A219788 * A219790 A219791 A219792

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Nov 28 2012

STATUS

approved