21953821 has 2 divisors, whose sum is σ = 21953822. Its totient is φ = 21953820.

The previous prime is 21953803. The next prime is 21953831. The reversal of 21953821 is 12835912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21483225 + 470596 = 4635^2 + 686^2 .

It is a cyclic number.

It is not a de Polignac number, because 21953821 - 2¹⁹ = 21429533 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21953831) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10976910 + 10976911.

It is an arithmetic number, because the mean of its divisors is an integer number (10976911).

Almost surely, 2^21953821 is an apocalyptic number.

21953821 is the 2505-th centered heptagonal number.

It is an amenable number.

21953821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21953821 is an equidigital number, since it uses as much as digits as its factorization.

21953821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4320, while the sum is 31.

The square root of 21953821 is about 4685.4904759267. The cubic root of 21953821 is about 280.0077421329.

The spelling of 21953821 in words is "twenty-one million, nine hundred fifty-three thousand, eight hundred twenty-one".