20987821 has 2 divisors, whose sum is σ = 20987822. Its totient is φ = 20987820.

The previous prime is 20987807. The next prime is 20987831. The reversal of 20987821 is 12878902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18826921 + 2160900 = 4339^2 + 1470^2 .

It is a cyclic number.

It is not a de Polignac number, because 20987821 - 2¹¹ = 20985773 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20987801) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10493910 + 10493911.

It is an arithmetic number, because the mean of its divisors is an integer number (10493911).

Almost surely, 2^20987821 is an apocalyptic number.

It is an amenable number.

20987821 is a deficient number, since it is larger than the sum of its proper divisors (1).

20987821 is an equidigital number, since it uses as much as digits as its factorization.

20987821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 16128, while the sum is 37.

The square root of 20987821 is about 4581.2466643917. The cubic root of 20987821 is about 275.8390725027.

The spelling of 20987821 in words is "twenty million, nine hundred eighty-seven thousand, eight hundred twenty-one".