21171720 has 32 divisors (see below), whose sum is σ = 63515520. Its totient is φ = 5645760.

The previous prime is 21171713. The next prime is 21171727. The reversal of 21171720 is 2717112.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21171713) and next prime (21171727).

It is not an unprimeable number, because it can be changed into a prime (21171727) 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, 88096 + ... + 88335.

It is an arithmetic number, because the mean of its divisors is an integer number (1984860).

Almost surely, 2^21171720 is an apocalyptic number.

21171720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21171720 is an abundant number, since it is smaller than the sum of its proper divisors (42343800).

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

21171720 is a wasteful number, since it uses less digits than its factorization.

21171720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 176445 (or 176441 counting only the distinct ones).

The product of its (nonzero) digits is 196, while the sum is 21.

The square root of 21171720 is about 4601.2737366951. The cubic root of 21171720 is about 276.6423810557.

Adding to 21171720 its reverse (2717112), we get a palindrome (23888832).

The spelling of 21171720 in words is "twenty-one million, one hundred seventy-one thousand, seven hundred twenty".