A121921 a(n) = A102897(n) - 1.

1, 3, 13, 121, 4959, 2771103, 151947502947

Offset: 0

Author

N. J. A. Sloane, Mar 17 2007

Keywords

• nonn
• hard
• more

Comments

Same as A102897, but counts nonempty set systems.

Number of Pi-systems of {1,...,n}. A Pi-system on a set X is a nonempty collection of subsets of X closed under finite intersections. - Matthew Azar, Jul 12 2025

References

• L. C. Evans and R. F. Gariepy, Measure Theory and Fine Properties of Functions, CRC Press, 2015, p. 7.

Crossrefs

Cf. A102897.