20991551 has 24 divisors (see below), whose sum is σ = 25485840. Its totient is φ = 17227392.

The previous prime is 20991533. The next prime is 20991577. The reversal of 20991551 is 15519902.

It is not a de Polignac number, because 20991551 - 2¹⁴ = 20975167 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20991511) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 153155 + ... + 153291.

It is an arithmetic number, because the mean of its divisors is an integer number (1061910).

Almost surely, 2^20991551 is an apocalyptic number.

20991551 is a deficient number, since it is larger than the sum of its proper divisors (4494289).

20991551 is a wasteful number, since it uses less digits than its factorization.

20991551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 263 (or 256 counting only the distinct ones).

The product of its (nonzero) digits is 4050, while the sum is 32.

The square root of 20991551 is about 4581.6537407360. The cubic root of 20991551 is about 275.8554124362.

The spelling of 20991551 in words is "twenty million, nine hundred ninety-one thousand, five hundred fifty-one".