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Abundant cubes.
2

%I #15 Aug 14 2019 08:27:15

%S 216,1000,1728,2744,5832,8000,10648,13824,17576,21952,27000,46656,

%T 64000,74088,85184,110592,125000,140608,157464,175616,216000,287496,

%U 314432,343000,373248,438976,474552,512000,592704,681472,729000,778688,884736

%N Abundant cubes.

%C Abundant cubes can't be prime powers for obvious reasons. Hence all these numbers can be represented as a^3*b^3 for some coprime a and b. a^3*b^3 is the magic product of the following magic 3 X 3 multiplicative square: [a*b^2, 1, a^2*b; a^2, ab, b^2; b, a^2*b^2; a].

%H Amiram Eldar, <a href="/A124581/b124581.txt">Table of n, a(n) for n = 1..10000</a>

%H Christian Boyer, <a href="http://www.multimagie.com/English/Multiplicative.htm">The smallest possible multiplicative magic squares</a>.

%e 216 is in the sequence because 216=6^3 and the sum of the proper divisors of 216 is 108+72+54+...+3+2+1 > 216.

%p isA005101 := proc(n) if numtheory[sigma](n) > 2*n then RETURN(true) ; else RETURN(false) ; fi ; end : for n from 1 to 120 do if isA005101(n^3) then printf("%d,",n^3) ; fi ; od ; # _R. J. Mathar_, Jan 07 2007

%p with(numtheory): a:=proc(n) if sigma(n^3)>2*n^3 then n^3 else fi end: seq(a(n),n=1..110); # _Emeric Deutsch_, Jan 10 2007

%t Select[Range[100]^3, DivisorSigma[1, #] > 2# &] (* _Amiram Eldar_, Aug 14 2019 *)

%Y Intersection of A000578 and A005101.

%Y Cf. A111029 = magic products of 3 X 3 multiplicative magic squares.

%K nonn

%O 1,1

%A _Tanya Khovanova_, Dec 27 2006

%E More terms from _R. J. Mathar_ and _Emeric Deutsch_, Jan 07 2007