21781155 has 32 divisors (see below), whose sum is σ = 38423808. Its totient is φ = 10448000.

The previous prime is 21781139. The next prime is 21781157. The reversal of 21781155 is 55118712.

It is not a de Polignac number, because 21781155 - 2⁴ = 21781139 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21781157) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 16012 + ... + 17318.

It is an arithmetic number, because the mean of its divisors is an integer number (1200744).

Almost surely, 2^21781155 is an apocalyptic number.

21781155 is a deficient number, since it is larger than the sum of its proper divisors (16642653).

21781155 is a wasteful number, since it uses less digits than its factorization.

21781155 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1427.

The product of its digits is 2800, while the sum is 30.

The square root of 21781155 is about 4667.0284978774. The cubic root of 21781155 is about 279.2717256125.

Adding to 21781155 its reverse (55118712), we get a palindrome (76899867).

It can be divided in two parts, 2178 and 1155, that added together give a palindrome (3333).

The spelling of 21781155 in words is "twenty-one million, seven hundred eighty-one thousand, one hundred fifty-five".