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A235592
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Numbers k such that k*(k+1) - prime(k) is prime.
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11
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2, 3, 4, 5, 6, 8, 9, 11, 14, 15, 18, 19, 20, 21, 26, 27, 29, 34, 36, 37, 38, 41, 44, 45, 48, 53, 54, 57, 61, 62, 69, 70, 71, 85, 86, 87, 89, 90, 98, 99, 102, 105, 112, 114, 117, 119, 131, 134, 135, 136, 137, 141, 145, 147, 149, 150, 153, 156, 157, 162, 170, 171, 175, 176, 180, 183, 187, 189, 198, 200
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OFFSET
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1,1
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COMMENTS
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It is known that prime(k) <= k*(k+1) for any positive integer k. The conjecture in A235613 implies that the sequence has infinitely many terms.
Conjecture: This sequence contains infinitely many primes.
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LINKS
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EXAMPLE
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a(1) = 2 since 1*2 - prime(1) = 0 is not prime, but 2*3 - prime(2) = 3 is prime.
a(2) = 3 since 3*4 - prime(3) = 7 is prime.
a(3) = 4 since 4*5 - prime(4) = 13 is prime.
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MATHEMATICA
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n=0; Do[If[PrimeQ[k(k+1)-Prime[k]], n=n+1; Print[n, " ", k]], {k, 1, 200}]
Select[Range[200], PrimeQ[(#(#+1))-Prime[#]]&] (* Harvey P. Dale, Apr 10 2020 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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