721515521 has 2 divisors, whose sum is σ = 721515522. Its totient is φ = 721515520.

The previous prime is 721515493. The next prime is 721515523. The reversal of 721515521 is 125515127.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 589518400 + 131997121 = 24280^2 + 11489^2 .

It is an emirp because it is prime and its reverse (125515127) is a distict prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-721515521 is a prime.

Together with 721515523, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_37308440 and since 721515521 and 37308440 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 (721515523) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 360757760 + 360757761.

It is an arithmetic number, because the mean of its divisors is an integer number (360757761).

Almost surely, 2^721515521 is an apocalyptic number.

It is an amenable number.

721515521 is a deficient number, since it is larger than the sum of its proper divisors (1).

721515521 is an equidigital number, since it uses as much as digits as its factorization.

721515521 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3500, while the sum is 29.

The square root of 721515521 is about 26861.0409515342. The cubic root of 721515521 is about 896.9093662867.

The spelling of 721515521 in words is "seven hundred twenty-one million, five hundred fifteen thousand, five hundred twenty-one".