B_n counts the partitions of an n-set; equivalently Σ_k S(n,k). Exponential generating function e^{e^x − 1}; Dobiński's formula B_n = e^{−1} Σ k^n/k!.
B_n counts the partitions of an n-set; equivalently Σ_k S(n,k). Exponential generating function e^{e^x − 1}; Dobiński's formula B_n = e^{−1} Σ k^n/k!.