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Search: id:A091421
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| A091421 |
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Numbers n such that n#*2^n - 1 is prime, where n# is product of prime numbers (primorials). |
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+0
2
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| 1, 2, 3, 4, 6, 7, 8, 15, 19, 31, 68, 69, 78, 82, 162, 210, 524
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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1# = 2 2# = 2*3 = 6 3# = 2*3*5 = 30
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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]]] (Steinerberger)
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PROGRAM
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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)), print(i)))
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CROSSREFS
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Sequence in context: A096360 A039087 A093710 this_sequence A138936 A135604 A084826
Adjacent sequences: A091418 A091419 A091420 this_sequence A091422 A091423 A091424
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KEYWORD
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hard,nonn
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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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a(17) from Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Feb 06 2006
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