A087554 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

A087554

a(n) = smallest number k >= n such that nk + 1 is a prime.

1, 2, 4, 4, 6, 6, 10, 9, 12, 10, 18, 13, 24, 14, 16, 16, 18, 21, 22, 20, 22, 28, 26, 24, 28, 26, 28, 34, 32, 33, 36, 36, 34, 37, 42, 36, 40, 39, 48, 40, 42, 46, 46, 47, 48, 51, 50, 54, 52, 51, 56, 55, 56, 54, 58, 56, 58, 61, 60, 67, 66, 63, 66, 67, 68, 66, 70, 75, 70, 79, 72, 79

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Conjectures: (1) k < 2n. (2) For every r, there exists a number S, nr < S < n(r+1) such that nS + 1 is prime.

LINKS

Table of n, a(n) for n=1..72.

MAPLE

for n from 1 to 120 do k := n: while(not isprime(n*k+1)) do k := k+1:od:a[n] := k:od:seq(a[l], l=1..120); # Sascha Kurz

CROSSREFS

Cf. A034693.

Sequence in context: A232091 A364066 A164798 * A281072 A226951 A251557