21511978 has 4 divisors (see below), whose sum is σ = 32267970. Its totient is φ = 10755988.

The previous prime is 21511967. The next prime is 21511979. The reversal of 21511978 is 87911512.

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 only one way, i.e., 19210689 + 2301289 = 4383^2 + 1517^2 .

It is not an unprimeable number, because it can be changed into a prime (21511979) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5377993 + ... + 5377996.

Almost surely, 2^21511978 is an apocalyptic number.

21511978 is a deficient number, since it is larger than the sum of its proper divisors (10755992).

21511978 is a wasteful number, since it uses less digits than its factorization.

21511978 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10755991.

The product of its digits is 5040, while the sum is 34.

The square root of 21511978 is about 4638.1006888596. The cubic root of 21511978 is about 278.1165168982.

The spelling of 21511978 in words is "twenty-one million, five hundred eleven thousand, nine hundred seventy-eight".