21640385 has 12 divisors (see below), whose sum is σ = 28014588. Its totient is φ = 15952896.

The previous prime is 21640379. The next prime is 21640427. The reversal of 21640385 is 58304612.

It can be written as a sum of positive squares in 6 ways, for example, as 3857296 + 17783089 = 1964^2 + 4217^2 .

It is not a de Polignac number, because 21640385 - 2¹⁰ = 21639361 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 37217 + ... + 37793.

It is an arithmetic number, because the mean of its divisors is an integer number (2334549).

Almost surely, 2^21640385 is an apocalyptic number.

It is an amenable number.

21640385 is a deficient number, since it is larger than the sum of its proper divisors (6374203).

21640385 is an frugal number, since it uses more digits than its factorization.

21640385 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1172 (or 595 counting only the distinct ones).

The product of its (nonzero) digits is 5760, while the sum is 29.

The square root of 21640385 is about 4651.9227207683. The cubic root of 21640385 is about 278.6687872967.

Adding to 21640385 its reverse (58304612), we get a palindrome (79944997).

The spelling of 21640385 in words is "twenty-one million, six hundred forty thousand, three hundred eighty-five".