721721906 has 4 divisors (see below), whose sum is σ = 1082582862. Its totient is φ = 360860952.

The previous prime is 721721863. The next prime is 721721909. The reversal of 721721906 is 609127127.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 609127127 = 7 ⋅87018161.

It can be written as a sum of positive squares in only one way, i.e., 561927025 + 159794881 = 23705^2 + 12641^2 .

It is not an unprimeable number, because it can be changed into a prime (721721909) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 180430475 + ... + 180430478.

Almost surely, 2^721721906 is an apocalyptic number.

721721906 is a deficient number, since it is larger than the sum of its proper divisors (360860956).

721721906 is a wasteful number, since it uses less digits than its factorization.

721721906 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 360860955.

The product of its (nonzero) digits is 10584, while the sum is 35.

The square root of 721721906 is about 26864.8823931913. The cubic root of 721721906 is about 896.9948765713.

The spelling of 721721906 in words is "seven hundred twenty-one million, seven hundred twenty-one thousand, nine hundred six".