21661380 has 72 divisors (see below), whose sum is σ = 70768152. Its totient is φ = 5331456.

The previous prime is 21661357. The next prime is 21661397. The reversal of 21661380 is 8316612.

It can be written as a sum of positive squares in 4 ways, for example, as 427716 + 21233664 = 654^2 + 4608^2 .

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 2289 + ... + 6968.

It is an arithmetic number, because the mean of its divisors is an integer number (982891).

Almost surely, 2^21661380 is an apocalyptic number.

21661380 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21661380 is an abundant number, since it is smaller than the sum of its proper divisors (49106772).

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

21661380 is a wasteful number, since it uses less digits than its factorization.

21661380 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 9285 (or 9280 counting only the distinct ones).

The product of its (nonzero) digits is 1728, while the sum is 27.

The square root of 21661380 is about 4654.1787675163. The cubic root of 21661380 is about 278.7588775015.

Adding to 21661380 its reverse (8316612), we get a palindrome (29977992).

The spelling of 21661380 in words is "twenty-one million, six hundred sixty-one thousand, three hundred eighty".