A352258 Number of symmetric binary n X n matrices with no 2 X 2 submatrix of all 1s.

2, 7, 42, 399, 5614, 112221, 3102020, 116076057, 5774524092, 376068483351, 31643635513816, 3401292647423655, 462391295351625128, 78801283167350942685, 1775935516860530625139, 230933325874558862792569

Offset: 1

Author

Brendan McKay, Mar 09 2022

Keywords

• nonn
• hard

Comments

Equivalently, the number of labeled graphs (loops but not multiple edges allowed) with none of these subgraphs: 4-cycle, edge with a loop on each end, triangle with at least one loop.

Crossrefs

Row sums of A350189.

Cf. A345249 (traceless matrices).