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A236423 Numbers n such that m^2 + n^2/m^2 is prime for every m|n. 3
1, 2, 6, 10, 14, 26, 74, 94, 130, 134, 146, 170, 206, 326, 386, 466, 470, 634, 1094, 1354, 1570, 1654, 1766, 1966, 2174, 2766, 3046, 3254, 3274, 3446, 4006, 4174, 4666, 4754, 4954, 5086, 5774, 5834, 6046, 6866, 6926, 7114, 7466, 8854, 9046, 9494, 10006, 10126 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If n = x*y then x^2 + y^2 is a prime.

These n > 1 must be even and squarefree.

Conjecture: the set of such n is infinite.

The conjecture follows from, e.g., Schinzel's hypothesis H. - Charles R Greathouse IV, Jan 28 2014

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..1000

MATHEMATICA

Select[Range[10^4], (d = Divisors[#]^2; And @@ PrimeQ[d + #^2/d]) &]) (* Giovanni Resta, Jan 26 2014 *)

PROG

(PARI) isok(n) = sumdiv(n, d, isprime(d^2 + n^2/d^2)) == numdiv(n); \\ Michel Marcus, Jan 25 2014

(PARI) is(n)=if(n%4!=2, return(n==1)); my(f=factor(n)); if(vecmax(f[, 2])>1, return(0)); fordiv(f, m, if(!isprime(m^2+(n/m)^2), return(0)); if(m^2>n, break)); 1 \\ Charles R Greathouse IV, Jan 28 2014

CROSSREFS

Cf. A080715.

Subsequence of A005574. - Michel Marcus, Jun 03 2017

Sequence in context: A342641 A118369 A226829 * A082816 A074105 A329657

Adjacent sequences:  A236420 A236421 A236422 * A236424 A236425 A236426

KEYWORD

nonn

AUTHOR

Thomas Ordowski, Jan 25 2014

EXTENSIONS

More terms from Michel Marcus, Jan 25 2014

STATUS

approved

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Last modified August 16 00:08 EDT 2022. Contains 356150 sequences. (Running on oeis4.)