# [seqfan] Re: A001250, A001251, A001252, A001253 counting permutations

Max Alekseyev maxale at gmail.com
Sat May 19 18:13:07 CEST 2012

```I'm preparing a paper on these sequences and would like to double
check the related information given in David, Kendall, and Barton.
Could somebody please tell me what exactly is listed in this book (or
maybe even scan the relevant pages)?
Thanks,
Max

On Thu, May 3, 2012 at 12:37 AM, Sean A. Irvine <sairvin at xtra.co.nz> wrote:
> The sequences A001250, A001251, A001252, A001253 (and possibly a few others)
> count runs in permutations. Their current values appear to be taken from
> Table 7.4.2 in "Symmetric Function and Allied Tables" by David, Kendall, and
> Barton. For example, A001251, gives the number of permutations of 1,2,...,n
> such that the longest run (either ascending or descending) is precisely 3.
>
> I would like to give these sequences more precise titles and extend them in
> the OEIS. But, I have run into a problem. My computed values for these
> sequences differ from those in the original reference for n>=13. I computed
> my values by brute force so I am inclined to believe them, but it is always
> possible I have overlooked something. Given the book was published in 1966,
> it seems unlikely that the entire original table (which goes up to n=14) was
> computed by brute force, but I could find no obvious generating function or
> recurrence in the book or other explanation as to how they produced their
> table. It seems likely that such a recurrence should exist, but it eludes
> me.
>
> Here are my brute force numbers for permutations of length n. Each row sums
> to n! as expected. For the case l=2 (A001250) my numbers agree with the
> formula and entries in the OEIS, but for A001251, A001252, A001253 they do
> not.
>
> n l=0, l=1, l=2, l=3, etc...
> 1 [0, 1]
> 2 [0, 0, 2]
> 3 [0, 0, 4, 2]
> 4 [0, 0, 10, 12, 2]
> 5 [0, 0, 32, 70, 16, 2]
> 6 [0, 0, 122, 442, 134, 20, 2]
> 7 [0, 0, 544, 3108, 1164, 198, 24, 2]
> 8 [0, 0, 2770, 24216, 10982, 2048, 274, 28, 2]
> 9 [0, 0, 15872, 208586, 112354, 22468, 3204, 362, 32, 2]
> 10 [0, 0, 101042, 1972904, 1245676, 264538, 39420, 4720, 462, 36, 2]
> 11 [0, 0, 707584, 20373338, 14909340, 3340962, 514296, 64020, 6644, 574, 40,
> 2]
> 12 [0, 0, 5405530, 228346522, 191916532, 45173518, 7137818, 913440, 98472,
> 9024, 698, 44, 2]
> 13 [0, 0, 44736512, 2763212980, 2646100822, 652209564, 105318770, 13760472,
> 1523808, 145080, 11908, 834, 48, 2]
> 14 [0, 0, 398721962, 35926266244, 38932850396, 10024669626, 1649355338,
> 219040274, 24744720, 2419872, 206388, 15344, 982, 52, 2]
> 15 [0, 0, 3807514624, 499676669254, 609137502242, 163546399460, 27356466626,
> 3681354658, 422335056, 42129360, 3690960, 285180, 19380, 1142, 56, 2]
>
> I would appreciate either an independent verification of my numbers or some
> insight into a way of computing these numbers without recourse to brute
> force.
>
> Sean.
>
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