A280647 - OEIS

%I #9 Dec 23 2024 14:53:45

%S 31,32,105,111,128,130,149,167,173,191,192,475,483,484,491,509,524,

%T 530,534,545,546,550,556,559,584,590,592,597,614,619,624,628,637,641,

%U 665,668,692,701,725,743,750,760,781,793,809,824,836,837,843,852,861,864

%N Numbers k such that k^3 has an odd number of digits and the middle digit is 7.

%C The sequence of cubes starts: 29791, 32768, 1157625, 1367631, 2097152, 2197000, 3307949, 4657463, ...

%H Lars Blomberg, <a href="/A280647/b280647.txt">Table of n, a(n) for n = 1..10000</a>

%H Jeremy Gardiner, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2016-December/017135.html">Middle digit in cube numbers</a>, Seqfan Mailing list, Dec 12 2016.

%e 31^3 = 29(7)91, 191^3 = 696(7)871, 619^3 = 2371(7)6659.

%t Select[Range[865], OddQ[len=Length[IntegerDigits[#^3]]]&&Part[IntegerDigits[#^3], (len+1)/2]==7 &] (* _Stefano Spezia_, Oct 03 2023 *)

%Y Cf. A280640-A280646, A280648-A280649, A181354.

%Y See A279420-A279429 for a k^2 version.

%Y See A279430-A279431 for a k^2 version in base 2.

%K nonn,base,easy

%O 1,1

%A _Lars Blomberg_, Jan 07 2017