21081281 has 4 divisors (see below), whose sum is σ = 22702932. Its totient is φ = 19459632.

The previous prime is 21081259. The next prime is 21081287. The reversal of 21081281 is 18218012.

It is a happy number.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 9247681 + 11833600 = 3041^2 + 3440^2 .

It is a cyclic number.

It is not a de Polignac number, because 21081281 - 2⁶ = 21081217 is a prime.

It is a super-2 number, since 2×21081281² = 888840817201922, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21081287) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 810806 + ... + 810831.

It is an arithmetic number, because the mean of its divisors is an integer number (5675733).

Almost surely, 2^21081281 is an apocalyptic number.

It is an amenable number.

21081281 is a deficient number, since it is larger than the sum of its proper divisors (1621651).

21081281 is a wasteful number, since it uses less digits than its factorization.

21081281 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1621650.

The product of its (nonzero) digits is 256, while the sum is 23.

The square root of 21081281 is about 4591.4356142714. The cubic root of 21081281 is about 276.2479087766.

Adding to 21081281 its reverse (18218012), we get a palindrome (39299293).

The spelling of 21081281 in words is "twenty-one million, eighty-one thousand, two hundred eighty-one".