21661543 has 2 divisors, whose sum is σ = 21661544. Its totient is φ = 21661542.

The previous prime is 21661531. The next prime is 21661573. The reversal of 21661543 is 34516612.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21661543 - 2¹⁷ = 21530471 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21661573) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10830771 + 10830772.

It is an arithmetic number, because the mean of its divisors is an integer number (10830772).

Almost surely, 2^21661543 is an apocalyptic number.

21661543 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661543 is an equidigital number, since it uses as much as digits as its factorization.

21661543 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 4320, while the sum is 28.

The square root of 21661543 is about 4654.1962786286. The cubic root of 21661543 is about 278.7595767119.

The spelling of 21661543 in words is "twenty-one million, six hundred sixty-one thousand, five hundred forty-three".