20805781 has 4 divisors (see below), whose sum is σ = 20815200. Its totient is φ = 20796364.

The previous prime is 20805767. The next prime is 20805787. The reversal of 20805781 is 18750802.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 20805781 - 2²³ = 12417173 is a prime.

It is a super-2 number, since 2×20805781² = 865761046039922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20805787) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 600 + ... + 6478.

It is an arithmetic number, because the mean of its divisors is an integer number (5203800).

Almost surely, 2^20805781 is an apocalyptic number.

It is an amenable number.

20805781 is a deficient number, since it is larger than the sum of its proper divisors (9419).

20805781 is an equidigital number, since it uses as much as digits as its factorization.

20805781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9418.

The product of its (nonzero) digits is 4480, while the sum is 31.

The square root of 20805781 is about 4561.3354404165. The cubic root of 20805781 is about 275.0392494943.

The spelling of 20805781 in words is "twenty million, eight hundred five thousand, seven hundred eighty-one".