

A242834


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


1



0, 0, 6, 4, 0, 3, 0, 56, 0, 10, 0, 4, 0, 14, 0, 0, 0, 306, 0, 380, 0, 22, 0, 552, 0, 26, 0, 0, 0, 6, 0, 992, 0, 34, 0, 0, 0, 1722, 0, 1892, 0, 46, 0, 2256, 0, 0, 0, 0, 0, 2862, 0, 8, 0, 58, 0, 3540, 0, 62, 0, 0, 0, 0, 0, 4556, 0, 0, 0, 5112, 0, 74, 0, 0, 0, 0, 0, 6320, 0, 82, 0, 6972
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OFFSET

1,3


COMMENTS

Since the largest k where k*n/(k+n) is an integer is given by n*(n1), a(n) = 0 is definite.


LINKS

Table of n, a(n) for n=1..80.


EXAMPLE

1*3/(1+3) = 3/4 is not prime. 2*3/(2+3) = 6/5 is not prime. 3*3/(3+3) = 9/6 is not prime. 4*3/(4+3) = 12/7 is not prime. 5*3/(5+3) = 15/8 is not prime. 6*3/(6+3) = 2 is prime. Thus, a(3) = 6.


PROG

(PARI) a(n)=for(k=1, n*(n1), s=(k*n)/(k+n); if(floor(s)==s, if(ispseudoprime(s), return(k))))
n=1; while(n<100, print(a(n)); n+=1)


CROSSREFS

Cf. A002378.
Sequence in context: A136133 A200020 A011487 * A010495 A111310 A190575
Adjacent sequences: A242831 A242832 A242833 * A242835 A242836 A242837


KEYWORD

nonn


AUTHOR

Derek Orr, May 27 2014


STATUS

approved



