21636621 has 6 divisors (see below), whose sum is σ = 31252910. Its totient is φ = 14424408.

The previous prime is 21636619. The next prime is 21636647. The reversal of 21636621 is 12663612.

21636621 is a `hidden beast` number, since 2 + 1 + 6 + 36 + 621 = 666.

It can be written as a sum of positive squares in only one way, i.e., 17388900 + 4247721 = 4170^2 + 2061^2 .

It is not a de Polignac number, because 21636621 - 2¹ = 21636619 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21636661) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1202026 + ... + 1202043.

Almost surely, 2^21636621 is an apocalyptic number.

It is an amenable number.

21636621 is a deficient number, since it is larger than the sum of its proper divisors (9616289).

21636621 is a wasteful number, since it uses less digits than its factorization.

21636621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2404075 (or 2404072 counting only the distinct ones).

The product of its digits is 2592, while the sum is 27.

The square root of 21636621 is about 4651.5181392745. The cubic root of 21636621 is about 278.6526296954.

The spelling of 21636621 in words is "twenty-one million, six hundred thirty-six thousand, six hundred twenty-one".