721661 has 2 divisors, whose sum is σ = 721662. Its totient is φ = 721660.

The previous prime is 721631. The next prime is 721663. The reversal of 721661 is 166127.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 688900 + 32761 = 830^2 + 181^2 .

It is a cyclic number.

It is not a de Polignac number, because 721661 - 2⁶ = 721597 is a prime.

Together with 721663, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p₅₈₁₆₃ and since 721661 and 58163 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (721663) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360830 + 360831.

It is an arithmetic number, because the mean of its divisors is an integer number (360831).

2⁷²¹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

721661 is a deficient number, since it is larger than the sum of its proper divisors (1).

721661 is an equidigital number, since it uses as much as digits as its factorization.

721661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 504, while the sum is 23.

The square root of 721661 is about 849.5063272278. The cubic root of 721661 is about 89.6969643442.

Adding to 721661 its reverse (166127), we get a palindrome (887788).

The spelling of 721661 in words is "seven hundred twenty-one thousand, six hundred sixty-one".