20991905 has 16 divisors (see below), whose sum is σ = 27950400. Its totient is φ = 15005760.

The previous prime is 20991899. The next prime is 20991907. The reversal of 20991905 is 50919902.

It is not a de Polignac number, because 20991905 - 2¹⁶ = 20926369 is a prime.

It is not an unprimeable number, because it can be changed into a prime (20991907) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 11 + ... + 6479.

It is an arithmetic number, because the mean of its divisors is an integer number (1746900).

Almost surely, 2^20991905 is an apocalyptic number.

It is an amenable number.

20991905 is a deficient number, since it is larger than the sum of its proper divisors (6958495).

20991905 is a wasteful number, since it uses less digits than its factorization.

20991905 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6544.

The product of its (nonzero) digits is 7290, while the sum is 35.

The square root of 20991905 is about 4581.6923729120. The cubic root of 20991905 is about 275.8569630961.

It can be divided in two parts, 2099 and 1905, that added together give a palindrome (4004).

The spelling of 20991905 in words is "twenty million, nine hundred ninety-one thousand, nine hundred five".