This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A193987 #12 Jan 20 2025 03:51:43 %S A193987 1,2,6,12,120,60,7560,360,1260,2520,294053760,5040,128501493120, %T A193987 332640,110880,55440,106858629141264000,277200,188391763176048432000, %U A193987 720720,21621600,27935107200,1356699703068812438127792000,3603600,857656800,18632716502400,227026800,183783600 %N A193987 Least number k such that tau(tau(k)) = n. %C A193987 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 A193987 Cf. A010553 (tau(tau(n))), A000005, A002182, A005179. %K A193987 nonn %O A193987 1,2 %A A193987 _T. D. Noe_, Aug 10 2011 %E A193987 a(27) corrected by _Amiram Eldar_, Jan 20 2025