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A038767
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Numbers k for which k-th primorial + square of (k+1)-th prime is also a prime.
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1
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1, 2, 3, 4, 6, 13, 19, 28, 41, 66, 85, 371, 437, 726, 924, 1063, 3401
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OFFSET
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1,2
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COMMENTS
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r = prime(n+1)^2 is the smallest possible composite number that, if added to the n-th primorial, might give a prime.
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LINKS
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FORMULA
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EXAMPLE
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a(5)=6, 6th primorial is 30030, square of 7th prime is 289, sum gives 30319, a prime.
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MATHEMATICA
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Block[{a = {}, p = 1, q = 1}, Do[q = NextPrime[q]; If[PrimeQ[p + q^2], AppendTo[a, i]]; p *= q, {i, 1200}]; Rest[a] - 1] (* Michael De Vlieger, Jan 03 2021 *)
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PROG
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(PARI) isok(n) = ispseudoprime(prime(n+1)^2 + prod(j=1, n, prime(j))); \\ Michel Marcus, Aug 26 2019
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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