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A071868
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Number of k (1 <= k <= n) such that k^2+1 is prime.
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0
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1, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 11, 11, 11, 11, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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FORMULA
| Hardy and Littlewood conjectured that : a(n) ~ c* sqrt(n)/Log(n) where c = prod(p prime, 1 - (-1)^((p-1)/2)/(p-1) ) = 1, 3727...
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PROG
| (PARI) for(n=1, 200, print1(sum(i=1, n, if(isprime(i^2+1)-1, 0, 1)), ", "))
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CROSSREFS
| Cf. A005574, A002496.
Sequence in context: A102515 A066063 A123087 * A179390 A082447 A139789
Adjacent sequences: A071865 A071866 A071867 * A071869 A071870 A071871
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KEYWORD
| easy,nonn
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AUTHOR
| Benoit Cloitre (benoit7848c(AT)orange.fr), Jun 09 2002
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