721609 has 4 divisors (see below), whose sum is σ = 824704. Its totient is φ = 618516.

The previous prime is 721597. The next prime is 721613. The reversal of 721609 is 906127.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 906127 = 131 ⋅6917.

It is a cyclic number.

It is not a de Polignac number, because 721609 - 2⁵ = 721577 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (721619) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 51537 + ... + 51550.

It is an arithmetic number, because the mean of its divisors is an integer number (206176).

2⁷²¹⁶⁰⁹ is an apocalyptic number.

It is an amenable number.

721609 is a deficient number, since it is larger than the sum of its proper divisors (103095).

721609 is a wasteful number, since it uses less digits than its factorization.

721609 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 103094.

The product of its (nonzero) digits is 756, while the sum is 25.

The square root of 721609 is about 849.4757206654. The cubic root of 721609 is about 89.6948098912.

It can be divided in two parts, 72 and 1609, that added together give a square (1681 = 41²).

The spelling of 721609 in words is "seven hundred twenty-one thousand, six hundred nine".