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A264886
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Integers n such that A061720(n-1) + 1 or A061720(n-1) - 1 is prime.
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0
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1, 2, 3, 4, 5, 8, 9, 15, 25, 36, 57, 80, 81, 133, 225, 281, 282, 288, 343, 632, 653
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OFFSET
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1,2
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COMMENTS
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Are there any other squares in sequence?
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LINKS
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EXAMPLE
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a(3) = 3 because 2*3*5 - 2*3 - 1 = 23 is prime.
a(6) = 8 because 2*3*5*7*11*13*17*19 - 2*3*5*7*11*13*17 + 1 = 9189181 is prime.
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MATHEMATICA
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t = Differences[FoldList[Times, 1, Prime@ Range@ 1200]]; Select[Range@ 360, Or[PrimeQ[t[[# - 1]] + 1], PrimeQ[t[[# - 1]] - 1]] &] - 1 (* Michael De Vlieger, Nov 28 2015, after Alonso del Arte at A061720 *)
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PROG
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(PARI) a(n) = prod(k=1, n, prime(k));
for(n=0, 1e3, if(ispseudoprime(a(n)-a(n-1)-1) || ispseudoprime(a(n)-a(n-1)+1), print1(n, ", ")))
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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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STATUS
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approved
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