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Search: id:A157187
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| A157187 |
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Number of ways to write n as p*q-(p+q) with primes p<=q. |
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+0
4
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| 1, 1, 0, 2, 0, 1, 0, 1, 0, 1, 0, 2, 0, 0, 0, 2, 0, 1, 0, 1, 0, 1, 0, 2, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 3, 0, 0, 0, 2, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 3, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 4, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 2, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 2, 0, 1, 0, 1, 0
(list; graph; listen)
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OFFSET
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0,4
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COMMENT
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The only even number which can be written in the given way is n=0=2*2-(2+2), since if q an odd prime, pq-(p+q) is always odd.
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EXAMPLE
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a(0)=1 since 0=2*2-(2+2) is the only possibility.
a(1)=1 since 1=2*3-(2+3) is the only possibility.
a(2)=0 since 2 cannot be written as pq-(p+q) for primes p,q.
a(3)=2 since 3 = 2*5-(2+5) = 3*3-(3+3) are the two possibilities.
a(15437822399)=100 since p can be taken to be any of {13, 41, 43, 109, 113, 151, 181, 199, 271, 401, 613, 617, 661, 673, 859, 883, 919, 1021, 1123, 1201, 1249, 1471, 1801, 1871, 1951, 2003, 2269, 2647, 2731, 2861, 3169, 3511, 3571, 4159, 4999, 5281, 5881, 6007, 6427, 7057, 7393, 7481, 7841, 9241, 9521, 10193, 12241, 12377, 12853, 13729, 15401, 15913, 16831, 17551, 18701, 20593, 21169, 22051, 22441, 23801, 26951, 27541, 28051, 30577, 30941, 32341, 32401, 34273, 34651, 36037, 36721, 40801, 42043, 46411, 47521, 48049, 51481, 53857, 57331, 59671, 63649, 65521, 66529, 70687, 72931, 76441, 77617, 78541, 87517, 91631, 92401, 96097, 97241, 101921, 102103, 103951, 117811, 120121, 122401, 123553}.
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PROGRAM
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(PARI) A157187(n)={ local(c=0, L=sqrtint(n++)); fordiv( n, d, d>L&break; isprime(d+1) | next; isprime(n/d+1) & c++); c}
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CROSSREFS
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Sequence in context: A085854 A117145 A083912 this_sequence A152140 A104975 A106404
Adjacent sequences: A157184 A157185 A157186 this_sequence A157188 A157189 A157190
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KEYWORD
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nonn
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AUTHOR
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M. F. Hasler (MHasler(AT)univ-ag.fr), Mar 11 2009
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