21971918 has 4 divisors (see below), whose sum is σ = 32957880. Its totient is φ = 10985958.

The previous prime is 21971903. The next prime is 21971927. The reversal of 21971918 is 81917912.

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

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5492978 + ... + 5492981.

It is an arithmetic number, because the mean of its divisors is an integer number (8239470).

Almost surely, 2^21971918 is an apocalyptic number.

21971918 is a deficient number, since it is larger than the sum of its proper divisors (10985962).

21971918 is a wasteful number, since it uses less digits than its factorization.

21971918 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10985961.

The product of its digits is 9072, while the sum is 38.

The square root of 21971918 is about 4687.4212526719. The cubic root of 21971918 is about 280.0846597742.

It can be divided in two parts, 2 and 1971918, that multiplied together give a triangular number (3943836 = T₂₈₀₈).

The spelling of 21971918 in words is "twenty-one million, nine hundred seventy-one thousand, nine hundred eighteen".