21999978 has 192 divisors, whose sum is σ = 67925760. Its totient is φ = 5132160.

The previous prime is 21999977. The next prime is 22000001. The reversal of 21999978 is 87999912.

It is a Harshad number since it is a multiple of its sum of digits (54).

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21999973) by changing a digit.

It is a polite number, since it can be written in 95 ways as a sum of consecutive naturals, for example, 594576 + ... + 594612.

It is an arithmetic number, because the mean of its divisors is an integer number (353780).

Almost surely, 2^21999978 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21999978, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33962880).

21999978 is an abundant number, since it is smaller than the sum of its proper divisors (45925782).

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

21999978 is a wasteful number, since it uses less digits than its factorization.

21999978 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 90 (or 73 counting only the distinct ones).

The product of its digits is 734832, while the sum is 54.

The square root of 21999978 is about 4690.4134146150. The cubic root of 21999978 is about 280.2038396642.

It can be divided in two parts, 21 and 999978, that added together give a palindrome (999999).

The spelling of 21999978 in words is "twenty-one million, nine hundred ninety-nine thousand, nine hundred seventy-eight".