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A108814 Numbers k such that k^4 + 4 is semiprime. 4
3, 5, 15, 25, 55, 125, 205, 385, 465, 635, 645, 715, 1095, 1145, 1175, 1245, 1275, 1315, 1375, 1565, 1615, 1675, 1685, 1965, 2055, 2085, 2095, 2405, 2455, 2535, 2665, 2835, 2925, 3135, 3305, 3535, 3755, 3775, 4025, 4155, 4175, 4365, 4605, 4615, 4735, 4785 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Except for the first, all the terms above generate brilliant numbers.

Numbers n such that n - 1 + i and n + 1 + i are (twin) Gaussian primes, see Shanks. - Charles R Greathouse IV, Apr 20 2011

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

Daniel Shanks, A Note on Gaussian Twin Primes, Mathematics of Computation 14:70 (1960), pp. 201-203.

FORMULA

a(k) = A096012(k) + 1. (Because n^4+4 = ((n-1)^2+1)((n+1)^2+1).) - Jeppe Stig Nielsen, Feb 26 2016

MATHEMATICA

Select[Range[5000], PrimeOmega[#^4+4]==2&] (* Harvey P. Dale, Sep 07 2017 *)

PROG

(PARI) forstep(n=1, 1e5, 2, if(isprime(n^2-2*n+2) && isprime(n^2+2*n+2), print1(n", "))) \\ Charles R Greathouse IV, Apr 20 2011

(MAGMA) IsSemiprime:=func< n | &+[ k[2]: k in Factorization(n) ] eq 2 >; [ n: n in [1..5000] | IsSemiprime(n^4+4)]; // Vincenzo Librandi, Apr 20 2011

CROSSREFS

Cf. A001358, A057781, A096012.

Sequence in context: A146835 A146795 A166479 * A290296 A329662 A018272

Adjacent sequences:  A108811 A108812 A108813 * A108815 A108816 A108817

KEYWORD

nonn

AUTHOR

Jason Earls, Jul 10 2005

STATUS

approved

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Last modified March 7 18:04 EST 2021. Contains 341909 sequences. (Running on oeis4.)