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A243015 Least number k such that k*n/(k-n) is prime or 0 if no such number exists. 0
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified January 24 10:22 EST 2020. Contains 331193 sequences. (Running on oeis4.)