The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 18 08:46 EDT 2022. Contains 356204 sequences. (Running on oeis4.)