[seqfan] Re: A001495

[israel at math.ubc.ca]

This seems to be what Gupta's article calls gamma^*(n+1), the number of 
(n+1) x (n+1) symmetric 0-1 matrices with row sums 2 and first row 
(1,1,0,...). Thus A(3) = 3 because there are 3 symmetric 4 x 4 0-1 matrices 
with row sums 2 and first row 1 1 0 0, namely

1100
1001
0011
0110

1100
1010
0101
0011

and

1100
1100
0011
0011

Cheers,
Robert

On Aug 4 2013, David Newman wrote:

>This sequence is titled Number of Stochastic Matrices of Integers and it
>begins
>
>1,1,1,3,13,70.  I don't understand this.  It seems to me that for n=2 there
>are two matrices of non-negative integers with row sums 1 and column sums
>of 1, namely
>
>((1,0),(0,1)) and ((0,1),(1,0)). So a(2) should be 2, not 1.   There is a
>reference to an article called "Enumeration of Symmetric Matrices', but I
>don't have access to this.  Can someone explain to me what is being counted
>by this sequence?
>
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