21808081 has 2 divisors, whose sum is σ = 21808082. Its totient is φ = 21808080.

The previous prime is 21808051. The next prime is 21808091. The reversal of 21808081 is 18080812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17564481 + 4243600 = 4191^2 + 2060^2 .

It is a cyclic number.

It is not a de Polignac number, because 21808081 - 2¹³ = 21799889 is a prime.

It is a super-2 number, since 2×21808081² = 951184793805122, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21808051) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10904040 + 10904041.

It is an arithmetic number, because the mean of its divisors is an integer number (10904041).

Almost surely, 2^21808081 is an apocalyptic number.

It is an amenable number.

21808081 is a deficient number, since it is larger than the sum of its proper divisors (1).

21808081 is an equidigital number, since it uses as much as digits as its factorization.

21808081 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1024, while the sum is 28.

The square root of 21808081 is about 4669.9123118106. The cubic root of 21808081 is about 279.3867573756.

Adding to 21808081 its reverse (18080812), we get a palindrome (39888893).

The spelling of 21808081 in words is "twenty-one million, eight hundred eight thousand, eighty-one".