A280647 Numbers k such that k^3 has an odd number of digits and the middle digit is 7.

31, 32, 105, 111, 128, 130, 149, 167, 173, 191, 192, 475, 483, 484, 491, 509, 524, 530, 534, 545, 546, 550, 556, 559, 584, 590, 592, 597, 614, 619, 624, 628, 637, 641, 665, 668, 692, 701, 725, 743, 750, 760, 781, 793, 809, 824, 836, 837, 843, 852, 861, 864

Offset: 1

Author

Lars Blomberg, Jan 07 2017

Keywords

• nonn
• base
• easy

Comments

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

Examples

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

Links

• Lars Blomberg, Table of n, a(n) for n = 1..10000
• Jeremy Gardiner, Middle digit in cube numbers, Seqfan Mailing list, Dec 12 2016.

Crossrefs

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

See A279420-A279429 for a k^2 version.

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

Programs

Mathematica

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