21991385 has 16 divisors (see below), whose sum is σ = 28583184. Its totient is φ = 16146432.

The previous prime is 21991349. The next prime is 21991397. The reversal of 21991385 is 58319912.

It can be written as a sum of positive squares in 8 ways, for example, as 9339136 + 12652249 = 3056^2 + 3557^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21991385 is a prime.

It is a Smith number, since the sum of its digits (38) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 11669 + ... + 13421.

It is an arithmetic number, because the mean of its divisors is an integer number (1786449).

Almost surely, 2^21991385 is an apocalyptic number.

It is an amenable number.

21991385 is a deficient number, since it is larger than the sum of its proper divisors (6591799).

21991385 is a wasteful number, since it uses less digits than its factorization.

21991385 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1964.

The product of its digits is 19440, while the sum is 38.

The square root of 21991385 is about 4689.4973078146. The cubic root of 21991385 is about 280.1673531860.

The spelling of 21991385 in words is "twenty-one million, nine hundred ninety-one thousand, three hundred eighty-five".