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Numbers whose divisors have a mean abundancy index that is larger than 3.
2

%I #6 Jul 19 2024 14:28:18

%S 10886400,13305600,14515200,18144000,19958400,21772800,23587200,

%T 23950080,24192000,25401600,26611200,27216000,29030400,29937600,

%U 30481920,31449600,31933440,32659200,33264000,33868800,35380800,35925120,36288000,37739520,38102400,39312000,39916800

%N Numbers whose divisors have a mean abundancy index that is larger than 3.

%C Numbers k such that A374777(k)/A374778(k) > 3.

%C The numbers whose mean abundancy index of divisors is larger than 2 are in A245214.

%C The least odd term in this sequence is 84712751711029943302437712454902728115050897458369518458984375.

%H Amiram Eldar, <a href="/A374779/b374779.txt">Table of n, a(n) for n = 1..10000</a>

%e 10886400 is a term since A374777(10886400)/A374778(10886400) = 70644571/23514624 = 3.004... > 3.

%t f[p_, e_] := ((e+1)*p^2 - (e+2)*p + p^(-e))/((e+1)*(p-1)^2); s[1] = 1; s[n_] := Times @@ f @@@ FactorInteger[n]; Select[Range[4*10^7], s[#] > 3 &]

%o (PARI) is(n) = {my(f = factor(n)); prod(i = 1, #f~, p=f[i,1]; e=f[i,2]; (-2*p - e*p + p^2 + e*p^2 + p^(-e))/((e + 1)*(p - 1)^2)) > 3;}

%Y Subsequence of A245214.

%Y Cf. A068403, A374777, A374778.

%K nonn

%O 1,1

%A _Amiram Eldar_, Jul 19 2024