21663821 has 2 divisors, whose sum is σ = 21663822. Its totient is φ = 21663820.

The previous prime is 21663809. The next prime is 21663869. The reversal of 21663821 is 12836612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17522596 + 4141225 = 4186^2 + 2035^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21663821 is a prime.

It is a super-2 number, since 2×21663821² = 938642280640082, which contains 22 as substring.

It is equal to p_1369271 and since 21663821 and 1369271 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21663871) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10831910 + 10831911.

It is an arithmetic number, because the mean of its divisors is an integer number (10831911).

Almost surely, 2^21663821 is an apocalyptic number.

It is an amenable number.

21663821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21663821 is an equidigital number, since it uses as much as digits as its factorization.

21663821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3456, while the sum is 29.

The square root of 21663821 is about 4654.4409975850. The cubic root of 21663821 is about 278.7693481312.

The spelling of 21663821 in words is "twenty-one million, six hundred sixty-three thousand, eight hundred twenty-one".