OFFSET
1,2
LINKS
Jon E. Schoenfield, Table of n, a(n) for n = 1..505 (lists all terms < 10^5)
EXAMPLE
a(1)=1 because a(0) is not defined and 2*1 + 1 = 3 is prime;
a(2)=2 because 2*1*2 + 1 = 5 is prime;
a(3)=3 because 2*1*2*3 + 1 = 13 is prime;
a(4) is not 4 because 2*1*2*3*4 + 1 = 49 is not prime, but a(4)=5 works because 2*1*2*3*5 + 1 = 61 is prime.
MATHEMATICA
k = 2; a = {}; Do[If[PrimeQ[k n + 1], k = k n; AppendTo[a, n]], {n, 1, 3000}]; a (* Artur Jasinski *)
nxt[{p_, a_}]:=Module[{k=a+1}, While[!PrimeQ[p*k+1], k++]; {p*k, k}]; NestList[ nxt, {2, 1}, 60][[All, 2]] (* Harvey P. Dale, Aug 18 2021 *)
PROG
(Python)
from sympy import isprime
from itertools import count, islice
def agen(): # generator of terms
an, p = 1, 2
while True:
yield an
an = next(k for k in count(an+1) if isprime(p*k+1))
p *= an
print(list(islice(agen(), 58))) # Michael S. Branicky, Jan 13 2023
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
Artur Jasinski, Sep 19 2008
EXTENSIONS
Edited by N. J. A. Sloane, Sep 21 2017 following suggestions from Richard C. Schroeppel
STATUS
approved