21391947 has 6 divisors (see below), whose sum is σ = 30899492. Its totient is φ = 14261292.

The previous prime is 21391939. The next prime is 21391949. The reversal of 21391947 is 74919312.

It is not a de Polignac number, because 21391947 - 2³ = 21391939 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21391949) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1188433 + ... + 1188450.

Almost surely, 2^21391947 is an apocalyptic number.

21391947 is a deficient number, since it is larger than the sum of its proper divisors (9507545).

21391947 is a wasteful number, since it uses less digits than its factorization.

21391947 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2376889 (or 2376886 counting only the distinct ones).

The product of its digits is 13608, while the sum is 36.

The square root of 21391947 is about 4625.1429167108. The cubic root of 21391947 is about 277.5982802605.

The spelling of 21391947 in words is "twenty-one million, three hundred ninety-one thousand, nine hundred forty-seven".