21757421 has 6 divisors (see below), whose sum is σ = 25309710. Its totient is φ = 18649176.

The previous prime is 21757409. The next prime is 21757423. The reversal of 21757421 is 12475712.

It can be written as a sum of positive squares in only one way, i.e., 19758025 + 1999396 = 4445^2 + 1414^2 .

It is not a de Polignac number, because 21757421 - 2¹⁸ = 21495277 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21757423) 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, 221966 + ... + 222063.

It is an arithmetic number, because the mean of its divisors is an integer number (4218285).

Almost surely, 2^21757421 is an apocalyptic number.

It is an amenable number.

21757421 is a deficient number, since it is larger than the sum of its proper divisors (3552289).

21757421 is an equidigital number, since it uses as much as digits as its factorization.

21757421 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 444043 (or 444036 counting only the distinct ones).

The product of its digits is 3920, while the sum is 29.

The square root of 21757421 is about 4664.4850734031. The cubic root of 21757421 is about 279.1702518982.

The spelling of 21757421 in words is "twenty-one million, seven hundred fifty-seven thousand, four hundred twenty-one".