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A266918 Perfect power Löschian numbers. 1

%I #19 Nov 24 2020 03:38:30

%S 1,4,9,16,25,27,36,49,64,81,100,121,144,169,196,225,243,256,289,324,

%T 343,361,400,441,484,529,576,625,676,729,784,841,900,961,1024,1089,

%U 1156,1225,1296,1369,1444,1521,1600,1681,1728,1764,1849,1936,2025,2116,2187,2197,2209,2304,2401,2500

%N Perfect power Löschian numbers.

%C Inspired by A266836. See first comment in A266836.

%C Intersection of A001597 and A003136.

%C Obviously, this sequence contains all positive squares.

%C Perfect powers that are not the Löschian numbers are 8, 32, 125, 128, 216, 512, 1000, 1331, 2048, 2744, 3125, 3375, 4913, 5832, 7776, ...

%H Amiram Eldar, <a href="/A266918/b266918.txt">Table of n, a(n) for n = 1..10000</a>

%e 25 is a term because 25 = 5^2 = 5^2 + 5*0 + 0^2.

%e 27 is a term because 27 = 3^3 = 3^2 + 3*3 + 3^2.

%e 243 is a term because 243 = 3^5 = 9^2 + 9*9 + 9^2.

%e 343 is a term because 343 = 7^3 = 18^2 + 18*1 + 1^2.

%t fQ[n_] := n == 1 || GCD @@ FactorInteger[n][[All, 2]] > 1; gQ[n_] := Resolve[Exists[{x, y}, Reduce[n == x^2 + x y + y^2, {x, y}, Integers]]]; Select[Range@ 2500, fQ@# && gQ@# &] (* _Michael De Vlieger_, Jan 06 2016, after _Ant King_ at A001597 and _Jean-François Alcover_ at A003136 *)

%o (PARI) x='x+O('x^10^4); p=eta(x)^3/eta(x^3); for(n=0, 9999, if(polcoeff(p, n) != 0 && (ispower(n) || n==1), print1(n, ", ")));

%o (PARI) is(n) = (ispower(n) || n==1) && #bnfisintnorm(bnfinit(z^2+z+1), n);

%o for(n=0, 1e4, if(is(n), print1(n, ", ")));

%Y Cf. Loeschian numbers: A003136 (all), A266836 (2*k+1), A202822 (3*k+1), A260682 (6*k+1).

%Y Cf. A001597.

%K nonn

%O 1,2

%A _Altug Alkan_, Jan 06 2016

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Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)