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A243015
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Least number k such that k*n/(k-n) is prime or 0 if no such number exists.
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0
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2, 6, 0, 20, 0, 42, 0, 0, 0, 110, 0, 156, 0, 0, 0, 272, 0, 342, 0, 0, 0, 506, 0, 0, 0, 0, 0, 812, 0, 930, 0, 0, 0, 0, 0, 1332, 0, 0, 0, 1640, 0, 1806, 0, 0, 0, 2162, 0, 0, 0, 0, 0, 2756, 0, 0, 0, 0, 0, 3422, 0, 3660, 0, 0, 0, 0, 0, 0, 4970, 0, 5256, 0, 0, 0, 0, 0, 6162
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OFFSET
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1,1
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COMMENTS
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When a(n) is nonzero, a(n) = n*(n+1).
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LINKS
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FORMULA
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EXAMPLE
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2*k/(k-2) is an integer when k=1,2,3,6. The only prime is when k = 6 (2*6/(6-2) = 3 is prime). Thus a(2) = 6.
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PROG
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(PARI) a(n)=for(k=1, n*(n+1), if(k!=n, s=(k*n)/(k-n); if(floor(s)==s, if(ispseudoprime(s), return(k)))))
n=1; while(n<100, print1(a(n), ", "); n+=1)
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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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