21661404 has 12 divisors (see below), whose sum is σ = 50543304. Its totient is φ = 7220464.

The previous prime is 21661399. The next prime is 21661421. The reversal of 21661404 is 40416612.

21661404 = 1642² + 1643² + ... + 1649².

It is a tau number, because it is divible by the number of its divisors (12).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 902547 + ... + 902570.

It is an arithmetic number, because the mean of its divisors is an integer number (4211942).

Almost surely, 2^21661404 is an apocalyptic number.

It is an amenable number.

21661404 is an abundant number, since it is smaller than the sum of its proper divisors (28881900).

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

21661404 is a wasteful number, since it uses less digits than its factorization.

21661404 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1805124 (or 1805122 counting only the distinct ones).

The product of its (nonzero) digits is 1152, while the sum is 24.

The square root of 21661404 is about 4654.1813458438. The cubic root of 21661404 is about 278.7589804530.

It can be divided in two parts, 2166 and 1404, that added together give a triangular number (3570 = T₈₄).

The spelling of 21661404 in words is "twenty-one million, six hundred sixty-one thousand, four hundred four".