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A242933 Numbers n such that k*n/(k+n) is never prime for all k. 0
2, 16, 28, 36, 40, 50, 52, 64, 66, 70, 76, 78, 88, 92, 96, 100, 112, 116, 120, 124, 126, 130, 136, 144, 148, 154, 156, 160, 162, 170, 172, 176, 184, 186, 188, 190, 196, 204, 208, 210, 216, 220, 222, 232, 236, 238, 244, 246, 248, 250, 256, 260, 266, 268, 276, 280, 286 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n were an odd number, k*n/(k+n) would never be prime for all k; these are trivial and are not included. See A242932.

It is believed that numbers in A016742 (except 4) are members of this sequence.

LINKS

Table of n, a(n) for n=1..57.

EXAMPLE

16*k/(16+k) is an integer for k = 16, 48, 112, and 240. These k-values give the integers 8, 12, 14, and 15, respectively. Since none are prime, 16 is a member of this sequence.

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<300, if(a(2*n)==0, print1(2*n, ", ")); n+=1)

CROSSREFS

Cf. A242932.

Sequence in context: A108705 A113933 A127871 * A321308 A109210 A056707

Adjacent sequences:  A242930 A242931 A242932 * A242934 A242935 A242936

KEYWORD

nonn

AUTHOR

Derek Orr, May 27 2014

STATUS

approved

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Last modified May 13 23:41 EDT 2021. Contains 343868 sequences. (Running on oeis4.)