21027781 has 4 divisors (see below), whose sum is σ = 21179200. Its totient is φ = 20876364.

The previous prime is 21027767. The next prime is 21027791. The reversal of 21027781 is 18772012.

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.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21027781 is a prime.

It is a super-2 number, since 2×21027781² = 884335147567922, 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 (21027701) 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, 75501 + ... + 75778.

It is an arithmetic number, because the mean of its divisors is an integer number (5294800).

Almost surely, 2^21027781 is an apocalyptic number.

It is an amenable number.

21027781 is a deficient number, since it is larger than the sum of its proper divisors (151419).

21027781 is a wasteful number, since it uses less digits than its factorization.

21027781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 151418.

The product of its (nonzero) digits is 1568, while the sum is 28.

The square root of 21027781 is about 4585.6058487402. The cubic root of 21027781 is about 276.0140238271.

Adding to 21027781 its reverse (18772012), we get a palindrome (39799793).

The spelling of 21027781 in words is "twenty-one million, twenty-seven thousand, seven hundred eighty-one".