21172356 has 18 divisors (see below), whose sum is σ = 53519102. Its totient is φ = 7057440.

The previous prime is 21172343. The next prime is 21172357. The reversal of 21172356 is 65327112.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 16402500 + 4769856 = 4050^2 + 2184^2 .

It is a tau number, because it is divible by the number of its divisors (18).

It is not an unprimeable number, because it can be changed into a prime (21172357) 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 5 ways as a sum of consecutive naturals, for example, 294025 + ... + 294096.

Almost surely, 2^21172356 is an apocalyptic number.

It is an amenable number.

21172356 is an abundant number, since it is smaller than the sum of its proper divisors (32346746).

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

21172356 is a wasteful number, since it uses less digits than its factorization.

21172356 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 588131 (or 588126 counting only the distinct ones).

The product of its digits is 2520, while the sum is 27.

The square root of 21172356 is about 4601.3428474740. The cubic root of 21172356 is about 276.6451511470.

Adding to 21172356 its reverse (65327112), we get a palindrome (86499468).

It can be divided in two parts, 21172 and 356, that added together give a triangular number (21528 = T₂₀₇).

The spelling of 21172356 in words is "twenty-one million, one hundred seventy-two thousand, three hundred fifty-six".