721366661 has 4 divisors (see below), whose sum is σ = 763800012. Its totient is φ = 678933312.

The previous prime is 721366651. The next prime is 721366727. The reversal of 721366661 is 166663127.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 533841025 + 187525636 = 23105^2 + 13694^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-721366661 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (721366601) 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, 21216650 + ... + 21216683.

It is an arithmetic number, because the mean of its divisors is an integer number (190950003).

Almost surely, 2^721366661 is an apocalyptic number.

It is an amenable number.

721366661 is a deficient number, since it is larger than the sum of its proper divisors (42433351).

721366661 is a wasteful number, since it uses less digits than its factorization.

721366661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 42433350.

The product of its digits is 54432, while the sum is 38.

The square root of 721366661 is about 26858.2698809882. The cubic root of 721366661 is about 896.8476798745.

The spelling of 721366661 in words is "seven hundred twenty-one million, three hundred sixty-six thousand, six hundred sixty-one".