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A111925 Numbers of the form a^2 + b^4, with a,b > 0. 24

%I

%S 2,5,10,17,20,25,26,32,37,41,50,52,65,80,82,85,90,97,101,106,116,117,

%T 122,130,137,145,160,162,170,181,185,197,202,212,225,226,241,250,257,

%U 260,265,272,277,281,290,292,305,306,320,325,337,340,356,362,370,377

%N Numbers of the form a^2 + b^4, with a,b > 0.

%C Subsequence of A000404.

%C Although there are squares, cubes, fifth powers, ... in this sequence, there are no fourth powers. - _Altug Alkan_, Apr 09 2016

%C Also, numbers z such that z^5 = x^2 + y^4 for x, y >= 1. - _M. F. Hasler_, Apr 16 2018

%C The Friedlander-Iwaniec theorem states that there are infinitely many prime numbers in this sequence. These primes are in A028916. - _Bernard Schott_, Mar 09 2019

%H R. J. Mathar, <a href="/A111925/b111925.txt">Table of n, a(n) for n = 1..1000</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Friedlander%E2%80%93Iwaniec_theorem">Friedlander-Iwaniec theorem</a>

%e 25 = 3^2 + 2^4, so 25 is an element of the sequence.

%p isA111925 := proc(n)

%p local a,b ;

%p for a from 1 do

%p if a^4 >= n then

%p return false;

%p end if;

%p b := n-a^4 ;

%p if issqr(b) then

%p return true;

%p end if;

%p end do:

%p end proc:

%p A111925 := proc(n)

%p option remember;

%p if n = 1 then

%p 2;

%p else

%p for a from procname(n-1)+1 do

%p if isA111925(a) then

%p return a;

%p end if;

%p end do:

%p end if;

%p end proc: # _R. J. Mathar_, Apr 22 2013

%t With[{nn=60},Take[Union[First[#]^2+Last[#]^4&/@Tuples[Range[nn],2]],nn]] (* _Harvey P. Dale_, Jul 09 2014 *)

%o (PARI) list(lim)=my(v=List(),t); lim\=1; for(b=1,sqrtnint(lim-1,4), t=b^4; for(a=1,sqrtint(lim-t), listput(v,t+a^2))); Set(v) \\ _Charles R Greathouse IV_, Jun 07 2016

%o (PARI) is(n)=for(b=1,sqrtnint(n-1,4), if(issquare(n-b^4), return(1))); 0 \\ _Charles R Greathouse IV_, Jun 07 2016

%Y Cf. A055394, A022549; complement of A111909; subsequence of A000404.

%Y Cf. A028916 (subsequence of primes).

%K nonn

%O 1,1

%A _Stefan Steinerberger_, Nov 25 2005

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Last modified January 28 22:35 EST 2020. Contains 331328 sequences. (Running on oeis4.)