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 A158235 Numbers n whose square can be represented as a repdigit number in some base less than n. 9
 11, 20, 39, 40, 49, 78, 133, 247, 494, 543, 1086, 1218, 1629, 1651, 1729, 2172, 2289, 2715, 3097, 3258, 3458, 3801, 4171, 4344, 4503, 4578, 4887, 5187, 5430, 6194, 6231, 6867, 6916, 7303, 7540, 7563, 8342, 8645, 8773, 9139, 9156, 9291, 10374, 12103 (list; graph; refs; listen; history; text; internal format)
 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 Cf. A158245 (primitive terms), A158912 (four-digit repdigit numbers). Cf. A158236 (the bases), A158237 (the repdigit). Sequence in context: A160843 A153368 A068600 * A158245 A076851 A164576 Adjacent sequences:  A158232 A158233 A158234 * A158236 A158237 A158238 KEYWORD nice,nonn AUTHOR T. D. Noe, Mar 14 2009 EXTENSIONS Inequality edited by T. D. Noe, Mar 30 2009 STATUS approved

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Last modified January 19 14:53 EST 2020. Contains 331049 sequences. (Running on oeis4.)