21132721 has 2 divisors, whose sum is σ = 21132722. Its totient is φ = 21132720.

The previous prime is 21132719. The next prime is 21132733. The reversal of 21132721 is 12723112.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15492096 + 5640625 = 3936^2 + 2375^2 .

It is a cyclic number.

It is not a de Polignac number, because 21132721 - 2¹ = 21132719 is a prime.

Together with 21132719, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21138721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10566360 + 10566361.

It is an arithmetic number, because the mean of its divisors is an integer number (10566361).

Almost surely, 2^21132721 is an apocalyptic number.

It is an amenable number.

21132721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21132721 is an equidigital number, since it uses as much as digits as its factorization.

21132721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 168, while the sum is 19.

The square root of 21132721 is about 4597.0339350499. The cubic root of 21132721 is about 276.4724152171.

Adding to 21132721 its reverse (12723112), we get a palindrome (33855833).

The spelling of 21132721 in words is "twenty-one million, one hundred thirty-two thousand, seven hundred twenty-one".