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A326311 Least numbers k such that A004018(k) is nondecreasing.

%I A326311 #29 Sep 14 2019 16:40:23
%S A326311 0,1,2,4,5,10,13,17,20,25,50,65,85,125,130,145,170,185,205,221,250,
%T A326311 260,265,290,305,325,425,650,725,845,850,925,1025,1105,1625,1885,2125,
%U A326311 2210,2405,2465,2665,3145,3250,3445,3485,3625,3770,3965,4225,5525
%N A326311 Least numbers k such that A004018(k) is nondecreasing.
%C A326311 Least squared radius of a circle around a grid point of the square lattice such that the number of grid points on this circle is not smaller than the number of grid points on any circle around a grid point of the square lattice with smaller radius. a(1) = 0 by convention.
%H A326311 Hugo Pfoertner, <a href="/A326311/b326311.txt">Table of n, a(n) for n = 1..440</a>
%o A326311 (PARI) r2=0;for(k=0,6000,my(a004018 = if( k<1, k==0, 4 * sumdiv( k, d, (d%4==1) - (d%4==3))));if(a004018>=r2,r2=a004018;print1(k,", ")))
%o A326311 (Julia) using Nemo
%o A326311 function A326311List(len)
%o A326311     R, x = PolynomialRing(ZZ, "x")
%o A326311     e = theta_qexp(2, len, x)
%o A326311     L = [coeff(e, j) for j in 0:len - 1]
%o A326311     m = ZZ(0)
%o A326311     [n - 1 for (n, l) in enumerate(L) if l == (m = max(m, l))]
%o A326311 end
%o A326311 A326311List(1000) |> println # _Peter Luschny_, Sep 12 2019
%Y A326311 Cf. A004018, A071383, A071385, A326421.
%K A326311 nonn
%O A326311 1,3
%A A326311 _Hugo Pfoertner_, Sep 11 2019