A084435 a(1) = 2, then smallest prime of the form 2^k*a(n-1) + 1.
2, 3, 7, 29, 59, 1889, 3779, 7559, 4058207223809, 32465657790473, 4462046030502692971872257, 9582170887127842377060195852353537
Offset: 1
Keywords
Examples
a(3)=7 because 3*2+1=7 is prime; a(4)=29 because 7*2+1=15 is not prime, 7*4+1=29 is prime.
References
- Donald E. Knuth, The Art of Computer Programming, Vol. 2, Seminumerical Algorithms, problem 39, page 76.
Links
- Robert G. Wilson v, Table of n, a(n) for n = 1..15 (shortened by _N. J. A. Sloane_, Jan 13 2019)
Programs
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Mathematica
f[s_List] := Block[{k = 0, p = s[[-1]]}, While[q = 2^k*p + 1; !PrimeQ[ q], k++]; Append[s, q]]; s = {2}; Nest[f, s, 16] (* Robert G. Wilson v, Mar 11 2015 *) -
PARI
lista(nn) = {a = 2; print1(a, ", "); for (n=1, nn, k=0; while (!isprime(2^k*a+1), k++); a = 2^k*a+1; print1(a, ", "););} \\ Michel Marcus, Mar 18 2015
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