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

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));
a=concat(a, k2*a[terms]+1); terms++; used=setunion(used, Set([k2]))); a} (End)

Crossrefs

Sequence in context: A192397 A097396 A056788 * A085873 A303289 A051048

Adjacent sequences: A091856 A091857 A091858 * A091860 A091861 A091862

Keyword

nonn

Author

Amarnath Murthy, Mar 13 2004

Extensions

a(6) onwards by Rick L. Shepherd, Sep 08 2009

Status

approved