cp's OEIS Frontend

A190646 Least number k such that d(k-1) = d(k+1) = 2n or 0 if no such k exists, where d(n)=A000005(n).

%I A190646 #32 Jun 27 2025 21:59:40
%S A190646 4,7,19,41,127252,199,26890624,919,17299,6641,25269208984376,3401,
%T A190646 3900566650390624,640063,8418574,18089,1164385682220458984374,41651,
%U A190646 69528379848480224609374,128465,34084859374,12164095,150509919493198394775390626,90271
%N A190646 Least number k such that d(k-1) = d(k+1) = 2n or 0 if no such k exists, where d(n)=A000005(n).
%C A190646 a(28) = 2319679. a(30) = 3568049.
%C A190646 From _Chai Wah Wu_, Mar 13 2019: (Start)
%C A190646 a(26) = 64505245697, a(27) = 3959299, a(29) = 237828698392557762563228607177734374, a(31) = 26711406049549496732652187347412109374, a(32) = 441559, a(34) = 12535291248641, a(36) = 352351, a(37) = 1749348542212388688829378224909305572509765626, a(38) = 193405731995647.
%C A190646 Conjecture: if p is an odd prime, then a(p) is even.
%C A190646 (End)
%H A190646 Hugo van der Sanden, <a href="/A190646/b190646.txt">Table of n, a(n) for n = 1..40</a>
%e A190646 a(16)=18089 because d(18088)=d(18090)=2*16.
%Y A190646 Cf. A067888, A075036, A190355.
%K A190646 nonn
%O A190646 1,1
%A A190646 _Juri-Stepan Gerasimov_, May 15 2011, May 20 2011
%E A190646 a(7), a(11), a(13), and a(15) from _T. D. Noe_, May 25 2011
%E A190646 a(17), a(19), a(21)-a(23) from _Chai Wah Wu_, Mar 13 2019
%E A190646 b-file extending to a(40) from _Hugo van der Sanden_, Mar 04 2022