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URL: https://oeis.org/A091421

⇱ A091421 - OEIS

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A091421
Numbers m such that m#*2^m - 1 is prime, where m# = A002110(m).
2
1, 2, 3, 4, 6, 7, 8, 15, 19, 31, 68, 69, 78, 82, 162, 210, 524, 770, 1058, 1437, 1730, 3977, 5104, 8440
OFFSET
1,2
COMMENTS
1# = 2 2# = 2*3 = 6 3# = 2*3*5 = 30.
No more terms < 5000. - L. Joris Perrenet, Mar 17 2020
EXAMPLE
a(1) = 1 because 1#*2^1 - 1 = 3 is prime.
a(2) = 2 because 2#*2^2 - 1 = 23 is prime.
MATHEMATICA
For[n = 1, n < 60, n++, If[PrimeQ[2^n*Product[Prime[i], {i, 1, n}] - 1], Print[n]]] (* Stefan Steinerberger, Feb 06 2006 *)
PROG
(PARI) pp(n)= s=1; for(i=1, n, s=s*prime(i)); return(s);
f(n)=pp(n)!*2^n -1;
for (i=1, 500, if(isprime(f(i)), print1(i, ", ")))
CROSSREFS
Sequence in context: A096360 A039087 A093710 * A138936 A135604 A345437
KEYWORD
nonn,hard,more
AUTHOR
Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004
EXTENSIONS
a(17) from Stefan Steinerberger, Feb 06 2006
a(18)-a(22) from L. Joris Perrenet, Mar 17 2020
a(23)-a(24) from Michael S. Branicky, Aug 28 2024
STATUS
approved