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A316782 Number of achiral tree-factorizations of n. 13
1, 1, 1, 2, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 4, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 6, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 4, 1, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

A factorization of n is a finite nonempty multiset of positive integers greater than 1 with product n. An achiral tree-factorization of n is either (case 1) the number n itself or (case 2) a finite constant multiset of two or more achiral tree-factorizations, one of each factor in a factorization of n.

a(n) is also the number of ways to write n as a left-nested power-tower ((a^b)^c)^... of positive integers greater than one. For example, the a(64) = 6 ways are 64, 8^2, 4^3, 2^6, (2^3)^2, (2^2)^3.

a(n) depends only on the prime signature of n. - Andrew Howroyd, Nov 18 2018

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = 1 + Sum_{n = d^k, k>1} a(d).

a(p^n) = A067824(n) for prime p. - Andrew Howroyd, Nov 18 2018

EXAMPLE

The a(1296) = 4 achiral tree-factorizations are 1296, (36*36), (6*6*6*6), ((6*6)*(6*6)).

MATHEMATICA

a[n_]:=1+Sum[a[d], {d, n^(1/Rest[Divisors[GCD@@FactorInteger[n][[All, 2]]]])}];

Array[a, 100]

PROG

(PARI) a(n)={my(z, e=ispower(n, , &z)); 1 + if(e, sumdiv(e, d, if(d<e, a(z^d))))} \\ Andrew Howroyd, Nov 18 2018

CROSSREFS

Cf. A001055, A001597, A003238, A067824, A089723, A214577, A281118, A289078, A292504, A294336.

Sequence in context: A089723 A305253 A294336 * A326647 A326028 A294338

Adjacent sequences:  A316779 A316780 A316781 * A316783 A316784 A316785

KEYWORD

nonn

AUTHOR

Gus Wiseman, Jul 13 2018

STATUS

approved

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Last modified December 7 03:00 EST 2019. Contains 329836 sequences. (Running on oeis4.)