21217281 has 16 divisors (see below), whose sum is σ = 29917440. Its totient is φ = 13338000.

The previous prime is 21217277. The next prime is 21217289. The reversal of 21217281 is 18271212.

It is not a de Polignac number, because 21217281 - 2² = 21217277 is a prime.

It is a super-2 number, since 2×21217281² = 900346026065922, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21217289) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 13566 + ... + 15048.

It is an arithmetic number, because the mean of its divisors is an integer number (1869840).

It is a Proth number, since it is equal to 1295 ⋅ 2¹⁴ + 1 and 1295 < 2¹⁴.

Almost surely, 2^21217281 is an apocalyptic number.

It is an amenable number.

21217281 is a deficient number, since it is larger than the sum of its proper divisors (8700159).

21217281 is a wasteful number, since it uses less digits than its factorization.

21217281 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1756.

The product of its digits is 448, while the sum is 24.

The square root of 21217281 is about 4606.2219877032. The cubic root of 21217281 is about 276.8406813210.

Adding to 21217281 its reverse (18271212), we get a palindrome (39488493).

The spelling of 21217281 in words is "twenty-one million, two hundred seventeen thousand, two hundred eighty-one".