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 A091421 Numbers n such that n#*2^n - 1 is prime, where n# is product of prime numbers (primorials). 1
 1, 2, 3, 4, 6, 7, 8, 15, 19, 31, 68, 69, 78, 82, 162, 210, 524 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 1# = 2 2# = 2*3 = 6 3# = 2*3*5 = 30 LINKS 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]]] (Steinerberger) 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)), print(i))) CROSSREFS Sequence in context: A096360 A039087 A093710 * A138936 A135604 A084826 Adjacent sequences:  A091418 A091419 A091420 * A091422 A091423 A091424 KEYWORD hard,nonn AUTHOR Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004 EXTENSIONS a(17) from Stefan Steinerberger, Feb 06 2006 STATUS approved

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Last modified June 20 09:27 EDT 2019. Contains 324234 sequences. (Running on oeis4.)