A089485 - OEIS

A089485

Numbers k such that k^4 + 4^k = A001589(k) is a semiprime.

3, 5, 15, 35, 55

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OFFSET

1,1

COMMENTS

For n = 2*k + 1, n^4 + 4^n = (n^2 + n*2^(k + 1) + 2^n) * (n^2 - n*2^(k + 1) + 2^n) The sequence gives those values of n for which both parentheses are primes. No further terms were found for k<=5000.

a(6) > 120000, if it exists. - Tyler Busby, Feb 13 2023

LINKS

Table of n, a(n) for n=1..5.

Ignacio Larrosa Cañestro et al., Find all primes of the form 4^n + n^4. Discussion in newsgroup sci.math (2003).

EXAMPLE

a(1)=3 because 3^4+4^3=145=5*29, a(2)=5 because 5^4+4^5=1649=17*97.

MATHEMATICA

Select[Range[60], PrimeOmega[#^4+4^#]==2&] (* Harvey P. Dale, Jul 31 2020 *)

PROG

(PARI) for(k=0, 5000, my(n=2*k+1, p1=n^2+n*2^(k+1)+2^n, p2=n^2-n*2^(k+1)+2^n); if(ispseudoprime(p1)&&ispseudoprime(p2), print1(n, ", "))) \\ Hugo Pfoertner, Jul 24 2019

CROSSREFS

Cf. A001589.

Sequence in context: A148501 A148502 A295614 * A279684 A146212 A265762

Adjacent sequences: A089482 A089483 A089484 * A089486 A089487 A089488

KEYWORD

nonn,more

AUTHOR

Hugo Pfoertner, Nov 11 2003

STATUS

approved