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A249382
Smallest odd prime Q such that Q*2^prime(n)-1 is also a prime number.
3
3, 3, 7, 3, 3, 31, 7, 61, 13, 7, 43, 31, 19, 3, 31, 307, 733, 79, 43, 73, 421, 73, 181, 19, 157, 181, 3, 739, 421, 127, 103, 73, 571, 421, 109, 211, 1459, 103, 1693, 487, 829, 139, 1153, 439, 3067, 601, 199, 853, 421, 3541, 1069, 1279, 1297
OFFSET
1,1
EXAMPLE
3*2^2-1=11 prime so a(1)=3 as 2 is prime(1).
3*2^3-1=23 prime so a(2)=3 as 3 is prime(2).
3*2^5-1=95 composite.
5*2^5-1=159 composite.
7*2^5-1=223 prime so a(3)=7 as 5 is prime(3).
MATHEMATICA
a249382[n_Integer] := Module[{q = 2}, While[! PrimeQ[Prime[q]*2^Prime[n] - 1], q++]; Prime[q]]; a249382/@Range[53] (* Michael De Vlieger, Nov 12 2014 *)
PROG
(PFGW & SCRIPT)
SCRIPT
DIM i
DIM j
DIM n, 0
OPENFILEOUT myf, a(n)
LABEL loop1
SET n, n+1
SET i, 1
LABEL loop2
SET i, i+1
SET j, p(i)
PRP j*2^p(n)-1
IF ISPRP THEN GOTO a
GOTO loop2
LABEL a
WRITE myf, j
GOTO loop1
(PARI) listp(nn) = {for (n=1, nn, k=2; while(!isprime(prime(k)*2^prime(n)-1), k++); print1(prime(k), ", "); ); } \\ Michel Marcus, Oct 27 2014
CROSSREFS
Sequence in context: A359048 A096915 A249806 * A317929 A285387 A100803
KEYWORD
nonn
AUTHOR
Pierre CAMI, Oct 27 2014
STATUS
approved