721561 has 2 divisors, whose sum is σ = 721562. Its totient is φ = 721560.

The previous prime is 721547. The next prime is 721571. The reversal of 721561 is 165127.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 697225 + 24336 = 835^2 + 156^2 .

It is a cyclic number.

It is not a de Polignac number, because 721561 - 2⁵ = 721529 is a prime.

It is not a weakly prime, because it can be changed into another prime (721571) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360780 + 360781.

It is an arithmetic number, because the mean of its divisors is an integer number (360781).

2⁷²¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

721561 is a deficient number, since it is larger than the sum of its proper divisors (1).

721561 is an equidigital number, since it uses as much as digits as its factorization.

721561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 420, while the sum is 22.

The square root of 721561 is about 849.4474674752. The cubic root of 721561 is about 89.6928210734.

Adding to 721561 its reverse (165127), we get a palindrome (886688).

The spelling of 721561 in words is "seven hundred twenty-one thousand, five hundred sixty-one".