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Search: id:A071917
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| A071917 |
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Number of pairs (x,y) where x is even, y is odd, 1<=x<=n, 1<=y<=n and x+y is prime. |
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2
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| 0, 1, 2, 4, 5, 7, 9, 11, 14, 18, 21, 25, 28, 31, 35, 40, 44, 48, 52, 56, 61, 67, 72, 78, 84, 90, 97, 104, 110, 117, 124, 131, 138, 146, 154, 163, 172, 181, 190, 200, 209, 219, 228, 237, 247, 257, 266, 275, 285, 295, 306, 318, 329, 341, 354, 367, 381, 395, 408, 421
(list; graph; listen)
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OFFSET
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1,3
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FORMULA
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a(n) = sum over primes p from 3 to 2n-1 of min(p-1, 2n+1-p)/2.
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EXAMPLE
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a(6)=7: The sums x+y are 2+1, 2+3, 2+5, 4+1, 4+3, 6+1, 6+5.
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MATHEMATICA
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a[n_] := Sum[If[PrimeQ[p], Min[p-1, 2n+1-p]/2, 0], {p, 3, 2n-1}]
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CROSSREFS
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Sequence in context: A057352 A156625 A058577 this_sequence A051060 A116650 A140205
Adjacent sequences: A071914 A071915 A071916 this_sequence A071918 A071919 A071920
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KEYWORD
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easy,nonn
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AUTHOR
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Enoch Haga (Enokh(AT)comcast.net), Jun 13 2002
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EXTENSIONS
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Edited by Dean Hickerson (dean.hickerson(AT)yahoo.com), Jun 18 2002
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