21687281 has 8 divisors (see below), whose sum is σ = 27038784. Its totient is φ = 16899120.

The previous prime is 21687251. The next prime is 21687287. The reversal of 21687281 is 18278612.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21687281 - 2¹⁰ = 21686257 is a prime.

It is a super-2 number, since 2×21687281² = 940676314345922, 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 (21687287) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 140750 + ... + 140903.

It is an arithmetic number, because the mean of its divisors is an integer number (3379848).

Almost surely, 2^21687281 is an apocalyptic number.

It is an amenable number.

21687281 is a deficient number, since it is larger than the sum of its proper divisors (5351503).

21687281 is a wasteful number, since it uses less digits than its factorization.

21687281 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 281671.

The product of its digits is 10752, while the sum is 35.

The square root of 21687281 is about 4656.9604894180. The cubic root of 21687281 is about 278.8699393366.

It can be divided in two parts, 2168 and 7281, that added together give a palindrome (9449).

The spelling of 21687281 in words is "twenty-one million, six hundred eighty-seven thousand, two hundred eighty-one".