721561021 has 2 divisors, whose sum is σ = 721561022. Its totient is φ = 721561020.

The previous prime is 721561019. The next prime is 721561051. The reversal of 721561021 is 120165127.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 595896921 + 125664100 = 24411^2 + 11210^2 .

It is an emirp because it is prime and its reverse (120165127) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 721561021 - 2¹ = 721561019 is a prime.

Together with 721561019, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (721561051) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360780510 + 360780511.

It is an arithmetic number, because the mean of its divisors is an integer number (360780511).

Almost surely, 2^721561021 is an apocalyptic number.

It is an amenable number.

721561021 is a deficient number, since it is larger than the sum of its proper divisors (1).

721561021 is an equidigital number, since it uses as much as digits as its factorization.

721561021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 840, while the sum is 25.

The square root of 721561021 is about 26861.8878897221. The cubic root of 721561021 is about 896.9282194354.

The spelling of 721561021 in words is "seven hundred twenty-one million, five hundred sixty-one thousand, twenty-one".