21857621 has 2 divisors, whose sum is σ = 21857622. Its totient is φ = 21857620.

The previous prime is 21857617. The next prime is 21857651. The reversal of 21857621 is 12675812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21022225 + 835396 = 4585^2 + 914^2 .

It is a cyclic number.

It is not a de Polignac number, because 21857621 - 2² = 21857617 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21857651) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10928810 + 10928811.

It is an arithmetic number, because the mean of its divisors is an integer number (10928811).

Almost surely, 2^21857621 is an apocalyptic number.

It is an amenable number.

21857621 is a deficient number, since it is larger than the sum of its proper divisors (1).

21857621 is an equidigital number, since it uses as much as digits as its factorization.

21857621 is an evil number, because the sum of its binary digits is even.

The product of its digits is 6720, while the sum is 32.

The square root of 21857621 is about 4675.2134710620. The cubic root of 21857621 is about 279.5981522975.

The spelling of 21857621 in words is "twenty-one million, eight hundred fifty-seven thousand, six hundred twenty-one".