1,1

This sequence also holds for bases 5, 15, 20, 30, 40, 60 and 120. These bases (together with 10) are the proper divisors of 5! that are divisible by 5. - Carl R. White (oeisfan(AT)phodd.net), Jan 21 2008

The g.f. conjectured by S. Plouffe in 1992 dissertation is not correct; the first discrepancy is a(31) = 155, his g.f. gives 160. In fact, the g.f. for this sequence is not rational; the first differences are bounded but not periodic. [From Franklin T. Adams-Watters (FrankTAW(AT)Netscape.net), Jul 03 2009]

L. Moser, Problem 158, Math. Mag., 27 (1953), 54-55. Solution by C. W. Trigg.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

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

David Wells, The Penguin Dictionary of Curious and Interesting Numbers (Rev. ed. 1997), p. 42

T. D. Noe, Table of n, a(n) for n=1..1000

S. Plouffe, Approximations de S\'{e}ries G\'{e}n\'{e}ratrices et Quelques Conjectures, Dissertation, Universit\'{e} du Qu\'{e}bec \`{a} Montr\'{e}al, 1992.

S. Plouffe, 1031 Generating Functions and Conjectures, Universit\'{e} du Qu\'{e}bec \`{a} Montr\'{e}al, 1992.

N. J. A. Sloane, Transforms

Index entries for sequences related to factorial numbers

The simplest way to obtain this sequence is by constructing a power series A(x) = Sum_{ k >= 1} x^a(k) whose exponents give the terms of the sequence. Define e(n) = (5^n-1)/4, f(n) = (1-x^(e(n)-1))/(1-x^e(n-1)), t(n) = x^(e(n)-6).

Now use the recurrence A[2]=1 and for n>=3, A[n]=f(n)*A[n-1]+t(n); then A = limit_{n->infinity} x^5*A[n]. This follows easily from the explicit formula for A027868(n). Here is the beginning of A: x^5 + x^11 + x^17 + x^23 + x^29 + x^30 + x^36 + x^42 + x^48 + ... - N. J. A. Sloane (njas(AT)research.att.com), Feb 02 2007

Formula from C. W. Trigg (see the Moser reference): The terms are given by the formula r = 6k-1 + p_1[k/5] + p_2[k/5] + p_2[k/5^2] + p_3[k/5^3] + ... where k = 1 2 3 ... and p_i=1 except when [k/5^i] = k/5^i when p_i has the value (k-5^i)/k and [] denotes "floor".

17 is in the sequence because on passing from 74! to 75!, the number of end zeros jumps from 16 to 18 skipping 17.

More generally, we have:

n, n!

-----

0, 1

1, 1

2, 2

3, 6

4, 24

5, 120

6, 720

7, 5040

8, 40320

9, 362880

10, 3628800

11, 39916800

12, 479001600

13, 6227020800

14, 87178291200

15, 1307674368000

16, 20922789888000

17, 355687428096000

18, 6402373705728000

19, 121645100408832000

20, 2432902008176640000

21, 51090942171709440000

22, 1124000727777607680000

23, 25852016738884976640000

24, 620448401733239439360000

25, 15511210043330985984000000 <- jump from 4 to 6 trailing 0's, so 5 is a term

26, 403291461126605635584000000

27, 10888869450418352160768000000

28, 304888344611713860501504000000

29, 8841761993739701954543616000000

30, 265252859812191058636308480000000

etc.

read(transforms); e:=n->(5^n-1)/4; f:=n->(1-x^(e(n)-1))/(1-x^e(n-1)); t:=n->x^(e(n)-6); A[2]:=1; for n from 3 to 8 do A[n]:=f(n)*A[n-1]+t(n); od: POWERS(series(x^5*A[8], x, 5005), x, 5005); (N. J. A. Sloane, Feb 02 2007)

u=Union[FoldList[Plus, 0, IntegerExponent[Range[1000], 5]]]; Complement[Range[u[[ -1]]], u] (T. D. Noe, Feb 02 2007)

Cf. A000142, A027868, A080066 (first differences).

Sequence in context: A107745 A111863 A043389 this_sequence A031480 A070753 A038939

Adjacent sequences: A000963 A000964 A000965 this_sequence A000967 A000968 A000969

nonn,base,nice

N. J. A. Sloane (njas(AT)research.att.com), Robert G. Wilson v (rgwv(AT)rgwv.com)

More terms from Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 24 2003

Corrected by Sascha Kurz (sascha.kurz(AT)uni-bayreuth.de), Jan 27 2003