20728781 has 2 divisors, whose sum is σ = 20728782. Its totient is φ = 20728780.

The previous prime is 20728759. The next prime is 20728783. The reversal of 20728781 is 18782702.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13942756 + 6786025 = 3734^2 + 2605^2 .

It is a cyclic number.

It is not a de Polignac number, because 20728781 - 2⁶ = 20728717 is a prime.

It is a super-2 number, since 2×20728781² = 859364723491922, which contains 22 as substring.

Together with 20728783, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20728783) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10364390 + 10364391.

It is an arithmetic number, because the mean of its divisors is an integer number (10364391).

Almost surely, 2^20728781 is an apocalyptic number.

It is an amenable number.

20728781 is a deficient number, since it is larger than the sum of its proper divisors (1).

20728781 is an equidigital number, since it uses as much as digits as its factorization.

20728781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 12544, while the sum is 35.

The square root of 20728781 is about 4552.8871060021. The cubic root of 20728781 is about 274.6995329841.

The spelling of 20728781 in words is "twenty million, seven hundred twenty-eight thousand, seven hundred eighty-one".