21717640 has 32 divisors (see below), whose sum is σ = 55846080. Its totient is φ = 7445952.

The previous prime is 21717637. The next prime is 21717643. The reversal of 21717640 is 4671712.

It is a Cunningham number, because it is equal to 279³+1.

It is an interprime number because it is at equal distance from previous prime (21717637) and next prime (21717643).

It is a Harshad number since it is a multiple of its sum of digits (28).

It is a junction number, because it is equal to n+sod(n) for n = 21717599 and 21717608.

It is not an unprimeable number, because it can be changed into a prime (21717643) 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, 38502 + ... + 39061.

It is an arithmetic number, because the mean of its divisors is an integer number (1745190).

Almost surely, 2^21717640 is an apocalyptic number.

21717640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21717640 is an abundant number, since it is smaller than the sum of its proper divisors (34128440).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21717640 is a wasteful number, since it uses less digits than its factorization.

21717640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 77581 (or 77577 counting only the distinct ones).

The product of its (nonzero) digits is 2352, while the sum is 28.

The square root of 21717640 is about 4660.2188789798. The cubic root of 21717640 is about 279.0000042822. Note that the first 5 decimals are identical.

The spelling of 21717640 in words is "twenty-one million, seven hundred seventeen thousand, six hundred forty".