21561666 has 32 divisors (see below), whose sum is σ = 50294400. Its totient is φ = 6159888.

The previous prime is 21561629. The next prime is 21561697. The reversal of 21561666 is 66616512.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 3181 + ... + 7296.

It is an arithmetic number, because the mean of its divisors is an integer number (1571700).

Almost surely, 2^21561666 is an apocalyptic number.

21561666 is an abundant number, since it is smaller than the sum of its proper divisors (28732734).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21561666 is a wasteful number, since it uses less digits than its factorization.

21561666 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10503 (or 10489 counting only the distinct ones).

The product of its digits is 12960, while the sum is 33.

The square root of 21561666 is about 4643.4541022821. The cubic root of 21561666 is about 278.3304818414.

The spelling of 21561666 in words is "twenty-one million, five hundred sixty-one thousand, six hundred sixty-six".