21317681 has 16 divisors (see below), whose sum is σ = 26707968. Its totient is φ = 16530000.

The previous prime is 21317669. The next prime is 21317683. The reversal of 21317681 is 18671312.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21317681 is a prime.

It is a super-2 number, since 2×21317681² = 908887046435522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21317683) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 18776 + ... + 19878.

It is an arithmetic number, because the mean of its divisors is an integer number (1669248).

Almost surely, 2^21317681 is an apocalyptic number.

It is an amenable number.

21317681 is a deficient number, since it is larger than the sum of its proper divisors (5390287).

21317681 is a wasteful number, since it uses less digits than its factorization.

21317681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1372.

The product of its digits is 2016, while the sum is 29.

The square root of 21317681 is about 4617.1074278167. The cubic root of 21317681 is about 277.2766636334.

Adding to 21317681 its reverse (18671312), we get a palindrome (39988993).

It can be divided in two parts, 2 and 1317681, that multiplied together give a palindrome (2635362).

The spelling of 21317681 in words is "twenty-one million, three hundred seventeen thousand, six hundred eighty-one".