Sequences

a(n) = (n-1)!! - (n-2)!!.

Quantum factorials: (n-1)!! - (n-2)!! (mod n).

Squarefree part of n: a(n) is the smallest positive number m such that n/m is a square.

Erroneous version of A048798.

Duplicate of A004709.

Numbers that are not perfect powers.

Version 1 of the "previous prime" function: largest prime <= n.

Least prime >= n (version 1 of the "next prime" function).

Smallest k such that k*n is a double factorial.

Smallest number k such that n + k is prime.

Numbers that are not the difference of two primes.

Smallest k such that k!! is a multiple of n.

Lengths increase by 1, digits cycle through positive digits.

The number n written using the greedy algorithm in the base where the values of the places are 1 and primes.

a(n) = n^(n+1) - (n+1)^n.

Some permutation of digits is a factorial number.

Some nontrivial permutation of digits is a factorial number.

Numbers containing an even digit.

Odd numbers containing an even digit.

Some nontrivial permutation of digits gives an even number.

Numbers that contain only 1's and 2's. Nonempty binary strings of length n in lexicographic order.

Numbers that contain only 1's, 2's and 3's.

Some permutation of digits is a prime.

Some nontrivial permutation of digits is a prime.

Composite numbers such that some permutation of digits is a prime.

Some permutation of digits is a square.

Nonsquares such that some permutation of digits is a square.

Some nontrivial permutation of digits is a square.

Some permutation of digits is a cube.

Noncubes such that some permutation of digits is a cube.

Some nontrivial permutation of digits is a cube.

Decimal concatenation of sequence (n, n-1, ..., 2, 1, 2, ..., n-1, n).

Concatenation of sequence (1,3,..,2n-1,2n,2n-2,..,2).

a(n) is the largest even number k such that 6, 8, ..., k are sums of 2 of first n odd primes.

Expansion of g.f. (2-x-2*x^2)/((1-x)*(1-x+x^2)).

a(n) = 6*(2*n+1)! / ((n!)^2*(n+3)).

Largest squarefree number dividing n: the squarefree kernel of n, rad(n), radical of n.

Largest cubefree number dividing n.

Greatest k such that 3^k divides n. Or, 3-adic valuation of n.

Binary sieve: delete every 2nd number, then every 4th, 8th, etc.

Ternary sieve: delete every 3rd number, then every 9th, 27th, etc.

Generated by a sieve: keep first number, drop every 2nd, keep first, drop every 3rd, keep first, drop every 4th, etc.

Digital sum (i.e., sum of digits) of n; also called digsum(n).

Product of decimal digits of n.

Product of divisors of n.

Product of the proper divisors of n.

Numbers that contain an odd digit.

Even numbers with at least one odd digit.

Some nontrivial permutation of digits is an odd number.

Positive numbers k with the property that some permutation of the digits of k is a triangular number.