721991 has 2 divisors, whose sum is σ = 721992. Its totient is φ = 721990.

The previous prime is 721979. The next prime is 721997. The reversal of 721991 is 199127.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 721991 - 2¹⁸ = 459847 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 (721997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360995 + 360996.

It is an arithmetic number, because the mean of its divisors is an integer number (360996).

2⁷²¹⁹⁹¹ is an apocalyptic number.

721991 is a deficient number, since it is larger than the sum of its proper divisors (1).

721991 is an equidigital number, since it uses as much as digits as its factorization.

721991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1134, while the sum is 29.

The square root of 721991 is about 849.7005354829. The cubic root of 721991 is about 89.7106344227.

The spelling of 721991 in words is "seven hundred twenty-one thousand, nine hundred ninety-one".