21387178 has 4 divisors (see below), whose sum is σ = 32080770. Its totient is φ = 10693588.

The previous prime is 21387167. The next prime is 21387193. The reversal of 21387178 is 87178312.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 19158129 + 2229049 = 4377^2 + 1493^2 .

It is a super-2 number, since 2×21387178² = 914822765607368, which contains 22 as substring.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5346793 + ... + 5346796.

Almost surely, 2^21387178 is an apocalyptic number.

21387178 is a deficient number, since it is larger than the sum of its proper divisors (10693592).

21387178 is a wasteful number, since it uses less digits than its factorization.

21387178 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10693591.

The product of its digits is 18816, while the sum is 37.

The square root of 21387178 is about 4624.6273363375. The cubic root of 21387178 is about 277.5776499957.

It can be divided in two parts, 2138 and 7178, that added together give a triangular number (9316 = T₁₃₆).

The spelling of 21387178 in words is "twenty-one million, three hundred eighty-seven thousand, one hundred seventy-eight".