21691788 has 24 divisors (see below), whose sum is σ = 51849840. Its totient is φ = 7054080.

The previous prime is 21691781. The next prime is 21691793. The reversal of 21691788 is 88719612.

It is not an unprimeable number, because it can be changed into a prime (21691781) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 21553 + ... + 22536.

It is an arithmetic number, because the mean of its divisors is an integer number (2160410).

Almost surely, 2^21691788 is an apocalyptic number.

It is an amenable number.

21691788 is an abundant number, since it is smaller than the sum of its proper divisors (30158052).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21691788 is a wasteful number, since it uses less digits than its factorization.

21691788 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 44137 (or 44135 counting only the distinct ones).

The product of its digits is 48384, while the sum is 42.

The square root of 21691788 is about 4657.4443635968. Note that the first 3 decimals coincide. The cubic root of 21691788 is about 278.8892560313.

The spelling of 21691788 in words is "twenty-one million, six hundred ninety-one thousand, seven hundred eighty-eight".