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A193987 Least number k such that tau(tau(k)) = n. 2

%I #7 Mar 30 2012 17:23:02

%S 1,2,6,12,120,60,7560,360,1260,2520,294053760,5040,128501493120,

%T 332640,110880,55440,106858629141264000,277200,188391763176048432000,

%U 720720,21621600,27935107200,1356699703068812438127792000,3603600,857656800,18632716502400,6983776800,183783600

%N Least number k such that tau(tau(k)) = n.

%C Here tau is the number of divisors function, A000005. Such a k always exists because an upper bound is 2^(2^n-1). For n < 19, and small composite numbers, terms can be found among the highly composite numbers, A002182. The b-file in A005179 is useful when tau^(-1)(n) is small.

%Y Cf. A010553 (tau(tau(n))).

%K nonn

%O 1,2

%A _T. D. Noe_, Aug 10 2011

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Last modified March 29 07:27 EDT 2024. Contains 371265 sequences. (Running on oeis4.)