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).

4, 7, 19, 41, 127252, 199, 26890624, 919, 17299, 6641, 25269208984376, 3401, 3900566650390624, 640063, 8418574, 18089, 1164385682220458984374, 41651, 69528379848480224609374, 128465, 34084859374, 12164095, 150509919493198394775390626, 90271

Offset

1,1

Comments

a(28) = 2319679. a(30) = 3568049.

From Chai Wah Wu, Mar 13 2019: (Start)

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.

Conjecture: if p is an odd prime, then a(p) is even.

(End)

Links

Hugo van der Sanden, Table of n, a(n) for n = 1..40

Example

a(16)=18089 because d(18088)=d(18090)=2*16.

Crossrefs

Cf. A067888, A075036, A190355.

Sequence in context: A182228 A352007 A182646 * A220011 A305034 A133264

Adjacent sequences: A190643 A190644 A190645 * A190647 A190648 A190649

Keyword

nonn

Author

Juri-Stepan Gerasimov, May 15 2011, May 20 2011

Extensions

a(7), a(11), a(13), and a(15) from T. D. Noe, May 25 2011

a(17), a(19), a(21)-a(23) from Chai Wah Wu, Mar 13 2019

b-file extending to a(40) from Hugo van der Sanden, Mar 4 2022

Status

approved