20990861 has 2 divisors, whose sum is σ = 20990862. Its totient is φ = 20990860.

The previous prime is 20990857. The next prime is 20990863. The reversal of 20990861 is 16809902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18541636 + 2449225 = 4306^2 + 1565^2 .

It is a cyclic number.

It is not a de Polignac number, because 20990861 - 2² = 20990857 is a prime.

Together with 20990863, 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 (20990863) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10495430 + 10495431.

It is an arithmetic number, because the mean of its divisors is an integer number (10495431).

Almost surely, 2^20990861 is an apocalyptic number.

It is an amenable number.

20990861 is a deficient number, since it is larger than the sum of its proper divisors (1).

20990861 is an equidigital number, since it uses as much as digits as its factorization.

20990861 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 7776, while the sum is 35.

The square root of 20990861 is about 4581.5784397956. The cubic root of 20990861 is about 275.8523899135.

The spelling of 20990861 in words is "twenty million, nine hundred ninety thousand, eight hundred sixty-one".