OFFSET
1,1
COMMENTS
Alternatively, numbers n such that n^2 = d*(b^k-1)/(b-1) for some b, d, and k with d < b < n. See Inkeri link.
It appears that 11^2 and 20^2 are the only squares representable as repunits having more than two "digits" in some base (see A208242).
Some bases, such as 313, appear many times. Why? See A158236 for the bases and A158237 for the repdigit.
Can a square have more than one representation? The representations of 11^2, 20^2, 40^2, and 1218^2 have more than 3 "digits". Is the list of such numbers finite?
A generalization of this problem, to "determine all perfect powers with identical digits in some basis", is briefly mentioned on page 6 of Waldschmidt's paper. - T. D. Noe, Mar 30 2009
From Bernard Schott, Aug 25 2017: (Start)
Some bases, such as 313, appear 10 times; others, such as 653, appear 9 times.
The reason is that for these bases b, we have 111_b = a * c^2 with a/b ~ 1/100. So for k such that 1 <= k <= floor(b/a)^(1/2), we can write: (a*k^2, a*k^2, a*k^2)_b = (k*a*c)^2. For instance,
111_313 = 3*181^2 and (3*k^2, 3*k^2, 3*k^2)_313 = (3*k*181)^2 = (543*k)^2, for k = 1 to 10.
111_653 = 7*247^2 and (7*k^2, 7*k^2, 7*k^2)_653 = (7*k*247)^2 = (1729*k)^2, for k = 1 to 9. (End)
Each term of this sequence except 11 has a square which can also be represented as a repdigit in some base greater than n, so they are also Brazilian repdigits with only two digits. - Bernard Schott, Aug 25 2017
LINKS
Michael De Vlieger and Michel Marcus, Table of n, a(n) for n = 1..100 (first 75 terms from Michel Marcus).
K. Inkeri, On the diophantine equation a * (x^n - 1) / (x-1) = y^m, Acta Arithmetica, XXI (1972).
Michel Waldschmidt, Open Diophantine problems, arXiv:math/0312440 [math.NT], 2003-2004.
EXAMPLE
11^2 = 11111 in base 3.
20^2 = 1111 in base 7.
39^2 = 333 in base 22.
40^2 = 4444 in base 7.
49^2 = 777 in base 18.
78^2 = (12)(12)(12) in base 22.
1218^2 = (21)(21)(21)(21) in base 41.
MATHEMATICA
Do[sq = n^2; Do[If[Length[Union[IntegerDigits[sq, b]]] == 1, Print[{n, sq, b, IntegerDigits[sq, b]}]], {b, 2, n}], {n, 10000}]
PROG
(PARI) isok(n) = {for (b=2, n-1, if (#Set(digits(n^2, b)) == 1, return (1)); ); return (0); } \\ Michel Marcus, Sep 06 2017
CROSSREFS
KEYWORD
nice,nonn
AUTHOR
T. D. Noe, Mar 14 2009
EXTENSIONS
Inequality edited by T. D. Noe, Mar 30 2009
STATUS
approved