20218991 has 8 divisors (see below), whose sum is σ = 21905520. Its totient is φ = 18661656.

The previous prime is 20218973. The next prime is 20218993. The reversal of 20218991 is 19981202.

It is a happy number.

It is not a de Polignac number, because 20218991 - 2⁶ = 20218927 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20218993) 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, 2405 + ... + 6798.

It is an arithmetic number, because the mean of its divisors is an integer number (2738190).

Almost surely, 2^20218991 is an apocalyptic number.

20218991 is a deficient number, since it is larger than the sum of its proper divisors (1686529).

20218991 is an frugal number, since it uses more digits than its factorization.

20218991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9242 (or 9216 counting only the distinct ones).

The product of its (nonzero) digits is 2592, while the sum is 32.

The square root of 20218991 is about 4496.5532355350. The cubic root of 20218991 is about 272.4288892247.

The spelling of 20218991 in words is "twenty million, two hundred eighteen thousand, nine hundred ninety-one".