21087281 has 2 divisors, whose sum is σ = 21087282. Its totient is φ = 21087280.

The previous prime is 21087277. The next prime is 21087289. The reversal of 21087281 is 18278012.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21077281 + 10000 = 4591^2 + 100^2 .

It is a cyclic number.

It is not a de Polignac number, because 21087281 - 2² = 21087277 is a prime.

It is a super-2 number, since 2×21087281² = 889346839945922, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (21087289) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10543640 + 10543641.

It is an arithmetic number, because the mean of its divisors is an integer number (10543641).

Almost surely, 2^21087281 is an apocalyptic number.

It is an amenable number.

21087281 is a deficient number, since it is larger than the sum of its proper divisors (1).

21087281 is an equidigital number, since it uses as much as digits as its factorization.

21087281 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 21087281 is about 4592.0889581976. The cubic root of 21087281 is about 276.2741141771.

The spelling of 21087281 in words is "twenty-one million, eighty-seven thousand, two hundred eighty-one".