21779991 has 12 divisors (see below), whose sum is σ = 31814640. Its totient is φ = 14356320.

The previous prime is 21779987. The next prime is 21780001. The reversal of 21779991 is 19997712.

It is not a de Polignac number, because 21779991 - 2² = 21779987 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21779971) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 12795 + ... + 14396.

It is an arithmetic number, because the mean of its divisors is an integer number (2651220).

Almost surely, 2^21779991 is an apocalyptic number.

21779991 is a deficient number, since it is larger than the sum of its proper divisors (10034649).

21779991 is a wasteful number, since it uses less digits than its factorization.

21779991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 27286 (or 27283 counting only the distinct ones).

The product of its digits is 71442, while the sum is 45.

The square root of 21779991 is about 4666.9037915946. The cubic root of 21779991 is about 279.2667506991.

It can be divided in two parts, 217 and 79991, that added together give a palindrome (80208).

The spelling of 21779991 in words is "twenty-one million, seven hundred seventy-nine thousand, nine hundred ninety-one".