This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A121921 #13 Jul 17 2025 00:39:59
%S A121921 1,3,13,121,4959,2771103,151947502947
%N A121921 a(n) = A102897(n) - 1.
%C A121921 Same as A102897, but counts nonempty set systems.
%C A121921 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
%D A121921 L. C. Evans and R. F. Gariepy, Measure Theory and Fine Properties of Functions, CRC Press, 2015, p. 7.
%Y A121921 Cf. A102897.
%K A121921 nonn,hard,more
%O A121921 0,2
%A A121921 _N. J. A. Sloane_, Mar 17 2007