21780020 has 24 divisors (see below), whose sum is σ = 46855368. Its totient is φ = 8499200.

The previous prime is 21780007. The next prime is 21780037. The reversal of 21780020 is 2008712.

It can be written as a sum of positive squares in 4 ways, for example, as 1240996 + 20539024 = 1114^2 + 4532^2 .

It is a Harshad number since it is a multiple of its sum of digits (20).

It is a junction number, because it is equal to n+sod(n) for n = 21779974 and 21780001.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 12461 + ... + 14100.

It is an arithmetic number, because the mean of its divisors is an integer number (1952307).

Almost surely, 2^21780020 is an apocalyptic number.

21780020 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21780020 is an abundant number, since it is smaller than the sum of its proper divisors (25075348).

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

21780020 is a wasteful number, since it uses less digits than its factorization.

21780020 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 26611 (or 26609 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 20.

The square root of 21780020 is about 4666.9068985785. The cubic root of 21780020 is about 279.2668746467.

Adding to 21780020 its reverse (2008712), we get a palindrome (23788732).

The spelling of 21780020 in words is "twenty-one million, seven hundred eighty thousand, twenty".