20897991 has 6 divisors (see below), whose sum is σ = 30186000. Its totient is φ = 13931988.

The previous prime is 20897983. The next prime is 20897993. The reversal of 20897991 is 19979802.

It is not a de Polignac number, because 20897991 - 2³ = 20897983 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20897993) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1160991 + ... + 1161008.

It is an arithmetic number, because the mean of its divisors is an integer number (5031000).

Almost surely, 2^20897991 is an apocalyptic number.

20897991 is a deficient number, since it is larger than the sum of its proper divisors (9288009).

20897991 is a wasteful number, since it uses less digits than its factorization.

20897991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2322005 (or 2322002 counting only the distinct ones).

The product of its (nonzero) digits is 81648, while the sum is 45.

The square root of 20897991 is about 4571.4320513380. The cubic root of 20897991 is about 275.4449699760. Note that the first 3 decimals are identical.

The spelling of 20897991 in words is "twenty million, eight hundred ninety-seven thousand, nine hundred ninety-one".