21006780 has 48 divisors (see below), whose sum is σ = 61918080. Its totient is φ = 5306688.

The previous prime is 21006779. The next prime is 21006793. The reversal of 21006780 is 8760012.

It is a self number, because there is not a number n which added to its sum of digits gives 21006780.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 8074 + ... + 10353.

It is an arithmetic number, because the mean of its divisors is an integer number (1289960).

Almost surely, 2^21006780 is an apocalyptic number.

21006780 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21006780 is an abundant number, since it is smaller than the sum of its proper divisors (40911300).

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

21006780 is a wasteful number, since it uses less digits than its factorization.

21006780 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 18458 (or 18456 counting only the distinct ones).

The product of its (nonzero) digits is 672, while the sum is 24.

The square root of 21006780 is about 4583.3153939043. The cubic root of 21006780 is about 275.9221057226.

Adding to 21006780 its reverse (8760012), we get a palindrome (29766792).

The spelling of 21006780 in words is "twenty-one million, six thousand, seven hundred eighty".