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A139670 Number of n X n symmetric binary matrices with all row sums 4. 4
1, 26, 820, 35150, 1944530, 133948836, 11234051976, 1127512146540, 133475706272700 (list; graph; refs; listen; history; text; internal format)
OFFSET

4,2

COMMENTS

From R. J. Mathar, 07 Apr 2017: (Start)

These are the row sums of the following triangle, which shows in row n and column t the number of symmetric n X n {0,1}-matrices with trace t and 4 ones in each row and each column, 0 <= t <= n:

0:    1;

1:    0,     0;

2:    0,     0,     0;

3:    0,     0,     0,     0;

4:    0,     0,     0,     0,     1;

5:    1,     0,    10,     0,    15,     0;

6:   15,     0,   270,     0,   465,     0,    70;

7:  465,     0,  9660,     0, 19355,     0,  5670,     0;

(End)

LINKS

Table of n, a(n) for n=4..12.

EXAMPLE

a(4) = 1:

  1 1 1 1

  1 1 1 1

  1 1 1 1

  1 1 1 1

CROSSREFS

Cf. A000085 (row sums 1), A000986 (row sums 2), A110040 (row sums 3).

Sequence in context: A180792 A091742 A160140 * A231282 A263945 A241871

Adjacent sequences:  A139667 A139668 A139669 * A139671 A139672 A139673

KEYWORD

nonn,more

AUTHOR

R. H. Hardin, Jun 12 2008

STATUS

approved

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Last modified August 18 00:56 EDT 2019. Contains 326059 sequences. (Running on oeis4.)