21315681 has 6 divisors (see below), whose sum is σ = 30789330. Its totient is φ = 14210448.

The previous prime is 21315661. The next prime is 21315689. The reversal of 21315681 is 18651312.

It can be written as a sum of positive squares in only one way, i.e., 17766225 + 3549456 = 4215^2 + 1884^2 .

It is not a de Polignac number, because 21315681 - 2⁵ = 21315649 is a prime.

It is a super-2 number, since 2×21315681² = 908716512987522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21315689) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1184196 + ... + 1184213.

It is an arithmetic number, because the mean of its divisors is an integer number (5131555).

Almost surely, 2^21315681 is an apocalyptic number.

It is an amenable number.

21315681 is a deficient number, since it is larger than the sum of its proper divisors (9473649).

21315681 is a wasteful number, since it uses less digits than its factorization.

21315681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2368415 (or 2368412 counting only the distinct ones).

The product of its digits is 1440, while the sum is 27.

The square root of 21315681 is about 4616.8908369161. The cubic root of 21315681 is about 277.2679921044.

Adding to 21315681 its reverse (18651312), we get a palindrome (39966993).

It can be divided in two parts, 2 and 1315681, that multiplied together give a palindrome (2631362).

The spelling of 21315681 in words is "twenty-one million, three hundred fifteen thousand, six hundred eighty-one".