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A193987 Least number k such that tau(tau(k)) = n. 2
1, 2, 6, 12, 120, 60, 7560, 360, 1260, 2520, 294053760, 5040, 128501493120, 332640, 110880, 55440, 106858629141264000, 277200, 188391763176048432000, 720720, 21621600, 27935107200, 1356699703068812438127792000, 3603600, 857656800, 18632716502400, 6983776800, 183783600 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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.

LINKS

Table of n, a(n) for n=1..28.

CROSSREFS

Cf. A010553 (tau(tau(n))).

Sequence in context: A259140 A075071 A250269 * A089423 A062349 A325504

Adjacent sequences:  A193984 A193985 A193986 * A193988 A193989 A193990

KEYWORD

nonn

AUTHOR

T. D. Noe, Aug 10 2011

STATUS

approved

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Last modified September 17 16:17 EDT 2021. Contains 347487 sequences. (Running on oeis4.)