A367992 - OEIS

A367992

Smallest nonprime that is the n-th prime plus a multiple of the (n-1)-st primorial.

4, 9, 35, 187, 221, 2323, 120137, 1021039, 19399403, 223092899, 6469693261, 200560490167, 7420738134851, 304250263527253, 39248283995010137, 614889782588491463, 65178316954380089519, 3845520700308425278201, 117288381359406970983337, 7858321551080267055879161

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..20.

Alon Amit, How do you find the smallest positive integer n so 7+30n is not a prime number?, Quora, November 22, 2023.

EXAMPLE

For n = 1, the n-th prime (2) plus a multiple m of the (n-1) primorial (1) is 2+m, giving the smallest nonprime, a(1)=4, when m=2.

For n = 4, the n-th prime (7) plus a multiple m of the (n-1) primorial (30) is 7+30m, giving the smallest nonprime, a(4)=187, when m=6.

MATHEMATICA

a[n_] := Module[{p = Prime[n], r = Product[Prime[i], {i, 1, n - 1}]}, While[p += r; PrimeQ[p]]; p]; Array[a, 20] (* Amiram Eldar, Dec 07 2023 *)

CROSSREFS