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A120225 a(n) is the minimal number k>n such that n+k and n*k+1 are primes. 6
2, 3, 4, 7, 6, 7, 10, 9, 14, 13, 18, 19, 24, 15, 16, 21, 24, 29, 22, 21, 22, 31, 30, 43, 28, 33, 34, 39, 32, 41, 36, 39, 34, 37, 66, 43, 60, 41, 50, 43, 42, 55, 46, 53, 52, 51, 50, 59, 52, 51, 56, 55, 56, 55, 58, 75, 74, 69, 68, 67, 66, 75, 74, 67, 86, 83, 70, 89, 70, 79, 102, 79 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Differs from A120224 in the first term.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

a(3)=4 because 3+4=7 and 3*4+1=13 are prime;

a(8)=9 because 8+9=17 and 8*9+1=73 are prime,

MATHEMATICA

Reap[Do[Do[If[PrimeQ[{n+x, n*x+1}]=={True, True}, Sow[x]; Break[]], {x, n+1, 1000}], {n, 120}]][[2, 1]]

mnk[n_]:=Module[{k=n+1}, While[!AllTrue[{n+k, n*k+1}, PrimeQ], k++]; k]; Array[ mnk, 80] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, May 15 2015 *)

CROSSREFS

Cf. A085063, A092945, A085063, A120223, A120224.

Sequence in context: A319651 A340632 A074846 * A247798 A130685 A210202

Adjacent sequences:  A120222 A120223 A120224 * A120226 A120227 A120228

KEYWORD

nonn

AUTHOR

Zak Seidov, Jun 10 2006

STATUS

approved

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Last modified November 30 08:19 EST 2021. Contains 349419 sequences. (Running on oeis4.)