21780816 has 80 divisors (see below), whose sum is σ = 59579520. Its totient is φ = 6842880.

The previous prime is 21780809. The next prime is 21780823. The reversal of 21780816 is 61808712.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21780809) and next prime (21780823).

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, 120246 + ... + 120426.

It is an arithmetic number, because the mean of its divisors is an integer number (744744).

Almost surely, 2^21780816 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21780816, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29789760).

21780816 is an abundant number, since it is smaller than the sum of its proper divisors (37798704).

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

21780816 is a wasteful number, since it uses less digits than its factorization.

21780816 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 324 (or 318 counting only the distinct ones).

The product of its (nonzero) digits is 5376, while the sum is 33.

The square root of 21780816 is about 4666.9921791235. The cubic root of 21780816 is about 279.2702767514.

The spelling of 21780816 in words is "twenty-one million, seven hundred eighty thousand, eight hundred sixteen".