21661405 has 32 divisors (see below), whose sum is σ = 28569600. Its totient is φ = 15724800.

The previous prime is 21661399. The next prime is 21661421. The reversal of 21661405 is 50416612.

It is not a de Polignac number, because 21661405 - 2³ = 21661397 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21661405.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 274156 + ... + 274234.

It is an arithmetic number, because the mean of its divisors is an integer number (892800).

Almost surely, 2^21661405 is an apocalyptic number.

It is an amenable number.

21661405 is a deficient number, since it is larger than the sum of its proper divisors (6908195).

21661405 is a wasteful number, since it uses less digits than its factorization.

21661405 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 205.

The product of its (nonzero) digits is 1440, while the sum is 25.

The square root of 21661405 is about 4654.1814532740. The cubic root of 21661405 is about 278.7589847426.

The spelling of 21661405 in words is "twenty-one million, six hundred sixty-one thousand, four hundred five".