212111

Appears in sequences

• Semiprimes of form p*q with p < q, such that 2^p - 1 == 0 (mod q).at n=21A179768
• Number of 2 X 2 matrices having all terms in {1,...,n} and determinant >=3n.at n=28A211063
• Positive numbers n such that (digitsum(n))^2 equals (product of digits(n))^3.at n=19A261778
• Underline all even terms; concatenations of underlined terms and of non-underlined terms both equal the concatenation of the entire sequence. This is the lexicographically earliest such sequence without duplicate terms and with an even digit among the first two terms.at n=32A307312
• List of co-Lyndon words on {1,2} sorted first by length and then lexicographically.at n=15A329318