login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A280642 Numbers k such that k^3 has an odd number of digits and the middle digit is 2. 3
5, 9, 103, 113, 133, 146, 151, 154, 165, 180, 198, 202, 470, 473, 493, 496, 504, 507, 521, 531, 538, 542, 566, 569, 581, 591, 593, 599, 612, 618, 620, 650, 654, 673, 681, 686, 703, 711, 715, 728, 729, 732, 740, 779, 801, 829, 841, 850, 855, 856, 857, 858, 874 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The sequence of cubes starts: 125, 729, 1092727, 1442897, 2352637, 3112136, 3442951, 3652264, ...

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.

EXAMPLE

5^3 = 1(2)5, 180^3 = 583(2)000, 618^3 = 2360(2)9032.

MATHEMATICA

a[n_]:=Part[IntegerDigits[n], (Length[IntegerDigits[n]]+1)/2];

Select[Range[0, 874], OddQ[Length[IntegerDigits[#^3]]] && a[#^3]==2 &] (* Indranil Ghosh, Mar 06 2017 *)

PROG

(PARI)

isok(k) = my(d=digits(k^3)); (#d%2 == 1) && (d[#d\2 +1] == 2);

for(k=0, 874, if(isok(k)==1, print1(k, ", "))); \\ Indranil Ghosh, Mar 06 2017

(Python)

i=0

j=1

while i<=874:

    n=str(i**3)

    l=len(n)

    if l%2 and n[(l-1)//2]=="2":

        print(str(i), end=', ')

        j+=1

    i+=1 # Indranil Ghosh, Mar 06 2017

CROSSREFS

Cf. A280640-A280641, A280643-A280649, A181354.

See A279420-A279429 for a k^2 version.

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

Sequence in context: A098097 A279707 A329002 * A222583 A222374 A097397

Adjacent sequences:  A280639 A280640 A280641 * A280643 A280644 A280645

KEYWORD

nonn,base,easy,changed

AUTHOR

Lars Blomberg, Jan 07 2017

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 7 19:23 EDT 2021. Contains 343652 sequences. (Running on oeis4.)