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A091421
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Numbers m such that m#*2^m - 1 is prime, where m# = A002110(m).
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1
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1, 2, 3, 4, 6, 7, 8, 15, 19, 31, 68, 69, 78, 82, 162, 210, 524, 770, 1058, 1437, 1730, 3977, 5104, 8440
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OFFSET
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1,2
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COMMENTS
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1# = 2 2# = 2*3 = 6 3# = 2*3*5 = 30.
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LINKS
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EXAMPLE
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a(1)=1 because 1#*2^1 - 1 = 3 is prime.
a(2)=2 because 2#*2^2 - 1 = 23 is prime.
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MATHEMATICA
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For[n = 1, n < 60, n++, If[PrimeQ[2^n*Product[Prime[i], {i, 1, n}] - 1], Print[n]]] (* Stefan Steinerberger, Feb 06 2006 *)
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PROG
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(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, ", ")))
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CROSSREFS
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KEYWORD
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nonn,hard,more,changed
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AUTHOR
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Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004
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EXTENSIONS
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STATUS
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approved
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