21891780 has 36 divisors (see below), whose sum is σ = 66405612. Its totient is φ = 5837760.

The previous prime is 21891761. The next prime is 21891787. The reversal of 21891780 is 8719812.

It can be written as a sum of positive squares in 2 ways, for example, as 5875776 + 16016004 = 2424^2 + 4002^2 .

It is a tau number, because it is divible by the number of its divisors (36).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is not an unprimeable number, because it can be changed into a prime (21891787) 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, 60631 + ... + 60990.

Almost surely, 2^21891780 is an apocalyptic number.

21891780 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21891780 is an abundant number, since it is smaller than the sum of its proper divisors (44513832).

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

21891780 is a wasteful number, since it uses less digits than its factorization.

21891780 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 121636 (or 121631 counting only the distinct ones).

The product of its (nonzero) digits is 8064, while the sum is 36.

The square root of 21891780 is about 4678.8652470444. The cubic root of 21891780 is about 279.7437281020.

It can be divided in two parts, 2189 and 1780, that added together give a square (3969 = 63²).

The spelling of 21891780 in words is "twenty-one million, eight hundred ninety-one thousand, seven hundred eighty".