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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Since the largest k where k*n/(k+n) is an integer is given by n*(n-1), 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*(n-1), 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

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Last modified April 3 14:34 EDT 2020. Contains 333197 sequences. (Running on oeis4.)