21381281 has 2 divisors, whose sum is σ = 21381282. Its totient is φ = 21381280.

The previous prime is 21381259. The next prime is 21381307. The reversal of 21381281 is 18218312.

It is an a-pointer prime, because the next prime (21381307) can be obtained adding 21381281 to its sum of digits (26).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12285025 + 9096256 = 3505^2 + 3016^2 .

It is a cyclic number.

It is not a de Polignac number, because 21381281 - 2¹⁰ = 21380257 is a prime.

It is a super-2 number, since 2×21381281² = 914318354401922, which contains 22 as substring.

It is equal to p_1352546 and since 21381281 and 1352546 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21381281.

It is not a weakly prime, because it can be changed into another prime (21381211) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10690640 + 10690641.

It is an arithmetic number, because the mean of its divisors is an integer number (10690641).

Almost surely, 2^21381281 is an apocalyptic number.

It is an amenable number.

21381281 is a deficient number, since it is larger than the sum of its proper divisors (1).

21381281 is an equidigital number, since it uses as much as digits as its factorization.

21381281 is an evil number, because the sum of its binary digits is even.

The product of its digits is 768, while the sum is 26.

The square root of 21381281 is about 4623.9897274972. The cubic root of 21381281 is about 277.5521358634.

Adding to 21381281 its reverse (18218312), we get a palindrome (39599593).

The spelling of 21381281 in words is "twenty-one million, three hundred eighty-one thousand, two hundred eighty-one".