A091859 - OEIS

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A091859

a(1) = 2, a(n) is the least prime of the form 2k*a(n-1)+1 where 2k is the least even number not used earlier.

2, 5, 31, 311, 3733, 59729, 1194581, 57339889, 1605516893, 22477236503, 1933042339259, 143045133105167, 2574812395893007, 87543621460362239, 700348971682897913, 58829313621363424693, 1411903526912722192633

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

OFFSET

1,1

LINKS

Table of n, a(n) for n=1..17.

EXAMPLE

a(2) = 5 = 2*2+1, But a(3) = 31 though 11= 5*2 +1 is also prime.

PROG

Contribution from Rick L. Shepherd, Sep 08 2009: (Start)

(PARI) {a=[2]; terms=1; used=Set();

while(terms<30, k2=0; until(isprime(k2*a[terms]+1),

until(setsearch(used, k2)==0, k2=k2+2));