21721873 has 2 divisors, whose sum is σ = 21721874. Its totient is φ = 21721872.

The previous prime is 21721871. The next prime is 21721879. The reversal of 21721873 is 37812712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14197824 + 7524049 = 3768^2 + 2743^2 .

It is a cyclic number.

It is not a de Polignac number, because 21721873 - 2¹ = 21721871 is a prime.

Together with 21721871, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21721871) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10860936 + 10860937.

It is an arithmetic number, because the mean of its divisors is an integer number (10860937).

Almost surely, 2^21721873 is an apocalyptic number.

It is an amenable number.

21721873 is a deficient number, since it is larger than the sum of its proper divisors (1).

21721873 is an equidigital number, since it uses as much as digits as its factorization.

21721873 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4704, while the sum is 31.

The square root of 21721873 is about 4660.6730200691. The cubic root of 21721873 is about 279.0181297983.

The spelling of 21721873 in words is "twenty-one million, seven hundred twenty-one thousand, eight hundred seventy-three".