721978 has 4 divisors (see below), whose sum is σ = 1082970. Its totient is φ = 360988.

The previous prime is 721961. The next prime is 721979. The reversal of 721978 is 879127.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 879127 = 461 ⋅1907.

It can be written as a sum of positive squares in only one way, i.e., 660969 + 61009 = 813^2 + 247^2 .

It is not an unprimeable number, because it can be changed into a prime (721979) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 180493 + ... + 180496.

2⁷²¹⁹⁷⁸ is an apocalyptic number.

721978 is a deficient number, since it is larger than the sum of its proper divisors (360992).

721978 is a wasteful number, since it uses less digits than its factorization.

721978 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 360991.

The product of its digits is 7056, while the sum is 34.

The square root of 721978 is about 849.6928856946. The cubic root of 721978 is about 89.7100959833.

The spelling of 721978 in words is "seven hundred twenty-one thousand, nine hundred seventy-eight".