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 A208242 Perfect powers y^q with y > 1 and q > 1 which are Brazilian repunits with three or more digits in some base. 4
 121, 343, 400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These three numbers are the only known solutions y^q of the Nagell-Ljunggren equation (b^m-1)/(b-1) = y^q with y > 1, q > 1, b > 1, m > 2. Yann Bugeaud and Maurice Mignotte propose two alternative conjectures: A) The Nagell-Ljunggren equation has only these three solutions. Considering the current state of our knowledge, this conjecture seems too ambitious, while the next one seems more reasonable. B) The Nagell-Ljunggren equation has only a finite number of solutions. This last conjecture is true if the abc conjecture is true (see article Bugeaud-Mignotte in link (p. 148). Consequence: 121 is the only known square of prime which is Brazilian. There are no other solutions for some base b < 10000. Some theorems and results about this equation: With the exception of the 3 known solutions, 1) for q = 2, there are no other solutions than 11^2 and 20^2, 2) there is no other solution if 3 divides m than 7^3, 3) there is no other solution if 4 divides m than 20^2. - Bernard Schott, Apr 29 2019 From David A. Corneth, Apr 29 2019: (Start) Intersection of A001597 and A053696. a(4) > 10^25 if it exists using constraints above. In the Nagell-Ljunggren equation, we need b > 2. If b = 2, we get y^q = 2^m - 1 which by Catalan's conjecture has no solutions (see A001597). (End) LINKS Y. Bugeaud and M. Mignotte, L'équation de Nagell-Ljunggren (x^n-1)/(x-1) = y^q", Enseign. Math. 48(2002), 147-168. EXAMPLE 121 = 11^2 =  (3^5 - 1)/ (3 - 1) = 11111_3. 343 =  7^3 = (18^3 - 1)/(18 - 1) =   111_18. 400 = 20^2 =  (7^4 - 1)/ (7 - 1) =  1111_7. PROG (PARI) is(n) = if(!ispower(n), return(0)); for(b=2, n-1, my(d=digits(n, b)); if(#d > 2 && vecmin(d)==1 && vecmax(d)==1, return(1))); 0 \\ Felix Fröhlich, Apr 29 2019 CROSSREFS Cf. A001597, A053696, A220571 (Brazilian composites), A307745 (similar but with digits > 1). Sequence in context: A068872 A203856 A216166 * A167721 A253321 A253328 Adjacent sequences:  A208239 A208240 A208241 * A208243 A208244 A208245 KEYWORD nonn,base,bref,more AUTHOR Bernard Schott, Jan 11 2013 EXTENSIONS Small edits to the name by Bernard Schott, Apr 30 2019 STATUS approved

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Last modified October 17 06:08 EDT 2019. Contains 328106 sequences. (Running on oeis4.)