The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A242931 Largest number k such that k*n/(k+n) is prime or 0 if no such k exists. 0
 0, 0, 6, 12, 0, 30, 0, 56, 0, 10, 0, 132, 0, 182, 0, 0, 0, 306, 0, 380, 0, 22, 0, 552, 0, 26, 0, 0, 0, 870, 0, 992, 0, 34, 0, 0, 0, 1406, 0, 0, 0, 1722, 0, 1892, 0, 46, 0, 2256, 0, 0, 0, 0, 0, 2862, 0, 8, 0, 58, 0, 3540, 0, 3782, 0, 0, 0, 0, 0, 4556, 0, 0, 0, 5112, 0, 5402 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Since the largest k that makes k*n/(k+n) an integer is n*(n-1), the zero terms are definite. LINKS EXAMPLE (6*3)/(6+3) = 2 is prime. Since 6 = 3*(3-1), 6 is the largest number that makes k*n/(k+n) an integer. Thus a(3) = 6. PROG (PARI) a(n)=for(k=n*(1-n), 0, 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. A242834, A002378. Sequence in context: A243017 A084342 A243045 * A341828 A110645 A064913 Adjacent sequences:  A242928 A242929 A242930 * A242932 A242933 A242934 KEYWORD nonn AUTHOR Derek Orr, May 27 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 | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 14 19:53 EDT 2021. Contains 343903 sequences. (Running on oeis4.)