A098778 a(n) is the least k such that (k*prime(n)#)^2 + 1 is prime, where prime(n)# is the n-th primorial.

1, 1, 3, 1, 1, 12, 7, 1, 13, 13, 7, 10, 13, 6, 4, 1, 6, 46, 12, 27, 15, 40, 31, 14, 17, 9, 26, 9, 7, 5, 23, 27, 26, 2, 5, 9, 5, 24, 17, 23, 26, 166, 110, 2, 24, 87, 6, 113, 116, 3, 140, 12, 93, 26, 2, 15, 63, 15, 2, 143, 19, 19, 27, 122, 26, 28, 206, 10

Offset

1,3

Links

Amiram Eldar, Table of n, a(n) for n = 1..500

Mathematica

a[n_] := Module[{k = 1, p = Product[Prime[i], {i, 1, n}]}, While[! PrimeQ[(k*p)^2 + 1], k++]; k]; Array[a, 70] (* Amiram Eldar, Aug 28 2021 *)

Prog

(PARI) a(n) = my(k=1, P=prod(k=1, n, prime(k))); while (!ispseudoprime(sqr(k*P)+1), k++); k; \\ Michel Marcus, Sep 12 2021

Crossrefs

Cf. A002110.

Sequence in context: A067402 A113340 A134523 * A078122 A128592 A156584

Adjacent sequences: A098775 A098776 A098777 * A098779 A098780 A098781

Keyword

nonn

Author

Pierre CAMI, Oct 04 2004

Status

approved