20947781 has 2 divisors, whose sum is σ = 20947782. Its totient is φ = 20947780.

The previous prime is 20947741. The next prime is 20947783. The reversal of 20947781 is 18774902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20557156 + 390625 = 4534^2 + 625^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20947781 is a prime.

It is a super-2 number, since 2×20947781² = 877619057647922, which contains 22 as substring.

Together with 20947783, 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 (20947783) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10473890 + 10473891.

It is an arithmetic number, because the mean of its divisors is an integer number (10473891).

Almost surely, 2^20947781 is an apocalyptic number.

It is an amenable number.

20947781 is a deficient number, since it is larger than the sum of its proper divisors (1).

20947781 is an equidigital number, since it uses as much as digits as its factorization.

20947781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 28224, while the sum is 38.

The square root of 20947781 is about 4576.8745886249. The cubic root of 20947781 is about 275.6635480491.

The spelling of 20947781 in words is "twenty million, nine hundred forty-seven thousand, seven hundred eighty-one".