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A243015 Least number k such that k*n/(k-n) is prime or 0 if no such number exists. 0

%I

%S 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,

%T 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,

%U 0,0,0,0,0,3422,0,3660,0,0,0,0,0,0,4970,0,5256,0,0,0,0,0,6162

%N Least number k such that k*n/(k-n) is prime or 0 if no such number exists.

%C a(n) is nonzero for n in A006093.

%C When a(n) is nonzero, a(n) = n*(n+1).

%F a(A006093(n)) = n*(n+1).

%e 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.

%o (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)))))

%o n=1;while(n<100,print1(a(n),", ");n+=1)

%Y Cf. A006093.

%K nonn

%O 1,1

%A _Derek Orr_, May 29 2014

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Last modified February 18 09:39 EST 2020. Contains 332011 sequences. (Running on oeis4.)