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A007426 d_4(n), or tau_4(n), the number of ordered factorizations of n as n = rstu.
(Formerly M3231)
24
1, 4, 4, 10, 4, 16, 4, 20, 10, 16, 4, 40, 4, 16, 16, 35, 4, 40, 4, 40, 16, 16, 4, 80, 10, 16, 20, 40, 4, 64, 4, 56, 16, 16, 16, 100, 4, 16, 16, 80, 4, 64, 4, 40, 40, 16, 4, 140, 10, 40, 16, 40, 4, 80, 16, 80, 16, 16, 4, 160, 4, 16, 40, 84, 16, 64, 4, 40, 16, 64, 4, 200, 4, 16, 40, 40, 16 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Inverse Moebius transform applied thrice to all 1's sequence; or, Dirichlet convolution of d(n) [ A000005 ].

Let n = Product p_i^e_i. tau (A000005) is tau_2, A007425 is tau_3, this sequence is tau_4, where tau_k(n) (also written as d_k(n)) = Product_i binomial(k-1+e_i, k-1) is the k-th Piltz function. It gives the number of ordered factorizations of n as a product of k terms.

Appears to equal the number of solid partitions of n that can be extended in exactly 4 ways to a solid partition of n+1 by adding one element. - Wouter Meeussen, Sep 11, 2004

Equals row sums of A127172. - Gary W. Adamson, Nov 05 2007

REFERENCES

A. Ivic, The Riemann Zeta-Function, Wiley, NY, 1985, see p. xv.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

T. D. Noe, Table of n, a(n) for n=1..10000

N. J. A. Sloane, Transforms

FORMULA

a(n)=sum(d dividing n, tau(d)*tau(n/d)) - Benoit Cloitre, May 12 2003

Dirichlet g.f.: zeta^4(x)

MAPLE

A007426 := proc(n) local e, j; e := ifactors(n)[2]: product(binomial(3+e[j][2], 3), j=1..nops(e)); end;

MATHEMATICA

tau[n_, 1] = 1; tau[n_, k_] := tau[n, k] = Plus @@ (tau[ #, k - 1] & /@ Divisors[n]); Table[ tau[n, 4], {n, 77}] (* Robert G. Wilson v *)

PROG

(PARI) for(n=1, 100, print1(sumdiv(n, k, sumdiv(k, x, numdiv(x))), ", "))

(PARI) a(n)=sumdiv(n, d, numdiv(n/d)*numdiv(d))

CROSSREFS

Cf. A007425.

Cf. A127172, A051731.

Sequence in context: A205549 A120395 A160723 * A050348 A134637 A078910

Adjacent sequences:  A007423 A007424 A007425 * A007427 A007428 A007429

KEYWORD

nonn,easy,mult

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Robert G. Wilson v, Nov 02 2005

STATUS

approved

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Last modified April 23 05:30 EDT 2014. Contains 240913 sequences.