21689821 has 2 divisors, whose sum is σ = 21689822. Its totient is φ = 21689820.

The previous prime is 21689803. The next prime is 21689827. The reversal of 21689821 is 12898612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12320100 + 9369721 = 3510^2 + 3061^2 .

It is a cyclic number.

It is not a de Polignac number, because 21689821 - 2⁷ = 21689693 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 (21689827) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844910 + 10844911.

It is an arithmetic number, because the mean of its divisors is an integer number (10844911).

Almost surely, 2^21689821 is an apocalyptic number.

It is an amenable number.

21689821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21689821 is an equidigital number, since it uses as much as digits as its factorization.

21689821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13824, while the sum is 37.

The square root of 21689821 is about 4657.2331914990. The cubic root of 21689821 is about 278.8808259325.

The spelling of 21689821 in words is "twenty-one million, six hundred eighty-nine thousand, eight hundred twenty-one".