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A182301 Composite numbers that are not perfect powers and whose prime factors have an equal number of digits in decimal representation. 2

%I #7 Apr 30 2012 13:52:40

%S 6,10,12,14,15,18,20,21,24,28,30,35,40,42,45,48,50,54,56,60,63,70,72,

%T 75,80,84,90,96,98,105,108,112,120,126,135,140,143,147,150,160,162,

%U 168,175,180,187,189,192,200,209,210,221,224,240,245,247,250,252,253,270

%N Composite numbers that are not perfect powers and whose prime factors have an equal number of digits in decimal representation.

%H Arkadiusz Wesolowski, <a href="/A182301/b182301.txt">Table of n, a(n) for n = 1..10000</a>

%e 30 = 2*3*5 belongs to this sequence, since its prime factors all have one digit and 30 is not a perfect power.

%t lst1 = {}; lst2 = {}; r = 270; Do[b = IntegerDigits[FactorInteger[n]]; If[! PrimeQ[n] && Length[b[[-1, 1]]] == Length[b[[1, 1]]], AppendTo[lst1, n]], {n, 4, r}]; lst2 = Complement[lst1, Union[Flatten[Table[n^i, {i, 2, Log[2, r]}, {n, 2, r^(1/i)}]]]]; lst2

%Y Subsequence of A199009.

%K base,nonn

%O 1,1

%A _Arkadiusz Wesolowski_, Apr 23 2012

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Last modified June 15 01:02 EDT 2024. Contains 373402 sequences. (Running on oeis4.)