21745621 has 8 divisors (see below), whose sum is σ = 23045400. Its totient is φ = 20482560.

The previous prime is 21745613. The next prime is 21745627. The reversal of 21745621 is 12654712.

It can be written as a sum of positive squares in 4 ways, for example, as 4223025 + 17522596 = 2055^2 + 4186^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21745621 - 2³ = 21745613 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21745621.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21745627) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 7956 + ... + 10333.

It is an arithmetic number, because the mean of its divisors is an integer number (2880675).

Almost surely, 2^21745621 is an apocalyptic number.

It is an amenable number.

21745621 is a deficient number, since it is larger than the sum of its proper divisors (1299779).

21745621 is a wasteful number, since it uses less digits than its factorization.

21745621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18359.

The product of its digits is 3360, while the sum is 28.

The square root of 21745621 is about 4663.2200248326. The cubic root of 21745621 is about 279.1197740301.

Subtracting from 21745621 its reverse (12654712), we obtain a palindrome (9090909).

It can be divided in two parts, 21745 and 621, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21745621 in words is "twenty-one million, seven hundred forty-five thousand, six hundred twenty-one".