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A091424
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Numbers n such that n#*2^n + 1 is prime, where n# is product of prime numbers (primorial).
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0
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1, 3, 4, 6, 10, 30, 31, 98, 156, 230, 432, 490, 1623, 1666
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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 = 5 is prime
a(2)=3 because 3#*2^3 + 1 = 241 is prime
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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)), print(i)))
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CROSSREFS
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KEYWORD
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nonn,hard,more
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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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