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

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

Offset

1,1

Comments

a(n) is nonzero for n in A006093.

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

Links

Table of n, a(n) for n=1..75.

Formula

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

Example

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.

Prog

(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)

Crossrefs

Cf. A006093.

Sequence in context: A321713 A057635 A269943 * A139717 A285119 A202535

Adjacent sequences: A243012 A243013 A243014 * A243016 A243017 A243018

Keyword

nonn

Author

Derek Orr, May 29 2014

Status

approved