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A062831
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Number of ways n can be expressed as the sum of a nonzero square and 1 or a prime.
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0
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0, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 2, 0, 2, 1, 1, 2, 2, 1, 2, 2, 1, 2, 1, 0, 2, 3, 2, 1, 2, 0, 3, 2, 0, 2, 1, 1, 4, 2, 1, 2, 2, 1, 2, 2, 1, 3, 2, 1, 2, 2, 2, 2, 3, 1, 3, 2, 0, 2, 2, 0, 4, 2, 0, 3, 3, 2, 4, 2, 1, 2, 3, 1, 1, 3, 1, 4, 2, 1, 3, 1, 2, 5, 3, 0, 3, 3, 2, 2, 2, 0, 4, 2, 1, 3, 2, 1, 4, 1, 1, 3, 3
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,6
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FORMULA
| Note that a(k^2)=0 or 1 since each prime can be written only in one way as a difference of squares: (n+b)^2-n^2=p where p is a prime, only if b^2+2nb=b(b+2n) is prime, only if b=1. In that case p=2n+1; since every prime is an odd number we get an 1 in the distribution of a(k^2) for each odd number which is prime.
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MATHEMATICA
| a[n_] := Length[Select[n-Range[1, Floor[Sqrt[n]]]^2, #==1||PrimeQ[ # ]&]]
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CROSSREFS
| Sequence in context: A002217 A157047 A059342 * A037828 A030419 A155052
Adjacent sequences: A062828 A062829 A062830 * A062832 A062833 A062834
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KEYWORD
| nonn
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AUTHOR
| Santi Spadaro (spados(AT)katamail.com), Jul 20 2001
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EXTENSIONS
| Corrected and extended by Dean Hickerson, Jul 26, 2001
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