21661312 has 64 divisors (see below), whose sum is σ = 45826560. Its totient is φ = 10183680.

The previous prime is 21661307. The next prime is 21661351. The reversal of 21661312 is 21316612.

It is a tau number, because it is divible by the number of its divisors (64).

It is a super-2 number, since 2×21661312² = 938424875122688, which contains 22 as substring.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 210253 + ... + 210355.

It is an arithmetic number, because the mean of its divisors is an integer number (716040).

Almost surely, 2^21661312 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 21661312, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22913280).

21661312 is an abundant number, since it is smaller than the sum of its proper divisors (24165248).

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

21661312 is a wasteful number, since it uses less digits than its factorization.

21661312 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 201 (or 189 counting only the distinct ones).

The product of its digits is 432, while the sum is 22.

The square root of 21661312 is about 4654.1714622476. The cubic root of 21661312 is about 278.7585858054.

Adding to 21661312 its reverse (21316612), we get a palindrome (42977924).

The spelling of 21661312 in words is "twenty-one million, six hundred sixty-one thousand, three hundred twelve".