690721 has 2 divisors, whose sum is σ = 690722. Its totient is φ = 690720.

The previous prime is 690719. The next prime is 690757. The reversal of 690721 is 127096.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 404496 + 286225 = 636^2 + 535^2 .

It is a cyclic number.

It is not a de Polignac number, because 690721 - 2¹ = 690719 is a prime.

Together with 690719, it forms a pair of twin primes.

It is an alternating number because its digits alternate between even and odd.

690721 is a lucky number.

It is equal to p₅₅₈₆₁ and since 690721 and 55861 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (691721) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 345360 + 345361.

It is an arithmetic number, because the mean of its divisors is an integer number (345361).

2⁶⁹⁰⁷²¹ is an apocalyptic number.

It is an amenable number.

690721 is a deficient number, since it is larger than the sum of its proper divisors (1).

690721 is an equidigital number, since it uses as much as digits as its factorization.

690721 is an odious number, because the sum of its binary digits is odd.

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

The square root of 690721 is about 831.0962639791. The cubic root of 690721 is about 88.3963270452.

The spelling of 690721 in words is "six hundred ninety thousand, seven hundred twenty-one".