21661000 has 32 divisors (see below), whose sum is σ = 50689080. Its totient is φ = 8664000.

The previous prime is 21660967. The next prime is 21661001. The reversal of 21661000 is 16612.

It can be written as a sum of positive squares in 4 ways, for example, as 4571044 + 17089956 = 2138^2 + 4134^2 .

It is not an unprimeable number, because it can be changed into a prime (21661001) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 9831 + ... + 11830.

Almost surely, 2^21661000 is an apocalyptic number.

21661000 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21661000 is an abundant number, since it is smaller than the sum of its proper divisors (29028080).

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

21661000 is a wasteful number, since it uses less digits than its factorization.

21661000 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21682 (or 21668 counting only the distinct ones).

The product of its (nonzero) digits is 72, while the sum is 16.

The square root of 21661000 is about 4654.1379438087. The cubic root of 21661000 is about 278.7572474270.

Adding to 21661000 its reverse (16612), we get a palindrome (21677612).

It can be divided in two parts, 216 and 61000, that added together give a palindrome (61216).

The spelling of 21661000 in words is "twenty-one million, six hundred sixty-one thousand".