721621 has 2 divisors, whose sum is σ = 721622. Its totient is φ = 721620.

The previous prime is 721619. The next prime is 721631. The reversal of 721621 is 126127.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 378225 + 343396 = 615^2 + 586^2 .

It is an emirp because it is prime and its reverse (126127) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 721621 - 2¹ = 721619 is a prime.

Together with 721619, 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 (721631) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360810 + 360811.

It is an arithmetic number, because the mean of its divisors is an integer number (360811).

2⁷²¹⁶²¹ is an apocalyptic number.

It is an amenable number.

721621 is a deficient number, since it is larger than the sum of its proper divisors (1).

721621 is an equidigital number, since it uses as much as digits as its factorization.

721621 is an odious number, because the sum of its binary digits is odd.

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

The square root of 721621 is about 849.4827838161. The cubic root of 721621 is about 89.6953070818.

Adding to 721621 its reverse (126127), we get a palindrome (847748).

The spelling of 721621 in words is "seven hundred twenty-one thousand, six hundred twenty-one".