21001081 has 2 divisors, whose sum is σ = 21001082. Its totient is φ = 21001080.

The previous prime is 21001049. The next prime is 21001087. The reversal of 21001081 is 18010012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18567481 + 2433600 = 4309^2 + 1560^2 .

It is a cyclic number.

It is not a de Polignac number, because 21001081 - 2⁵ = 21001049 is a prime.

It is a super-2 number, since 2×21001081² = 882090806337122, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21001087) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10500540 + 10500541.

It is an arithmetic number, because the mean of its divisors is an integer number (10500541).

Almost surely, 2^21001081 is an apocalyptic number.

It is an amenable number.

21001081 is a deficient number, since it is larger than the sum of its proper divisors (1).

21001081 is an equidigital number, since it uses as much as digits as its factorization.

21001081 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 16, while the sum is 13.

The square root of 21001081 is about 4582.6936402077. The cubic root of 21001081 is about 275.8971515204.

Adding to 21001081 its reverse (18010012), we get a palindrome (39011093).

The spelling of 21001081 in words is "twenty-one million, one thousand, eighty-one".