721645 has 8 divisors (see below), whose sum is σ = 875160. Its totient is φ = 571200.

The previous prime is 721631. The next prime is 721661. The reversal of 721645 is 546127.

721645 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It can be written as a sum of positive squares in 4 ways, for example, as 86436 + 635209 = 294^2 + 797^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 721645 - 2⁵ = 721613 is a prime.

It is a super-3 number, since 3×721645³ = 1127436460396233375, which contains 333 as substring.

It is an Ulam number.

721645 is a lucky number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 210 + ... + 1219.

It is an arithmetic number, because the mean of its divisors is an integer number (109395).

2⁷²¹⁶⁴⁵ is an apocalyptic number.

It is an amenable number.

721645 is a deficient number, since it is larger than the sum of its proper divisors (153515).

721645 is a wasteful number, since it uses less digits than its factorization.

721645 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1535.

The product of its digits is 1680, while the sum is 25.

The square root of 721645 is about 849.4969099414. The cubic root of 721645 is about 89.6963014466.

The spelling of 721645 in words is "seven hundred twenty-one thousand, six hundred forty-five".