21957409 has 2 divisors, whose sum is σ = 21957410. Its totient is φ = 21957408.

The previous prime is 21957373. The next prime is 21957431. The reversal of 21957409 is 90475912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18037009 + 3920400 = 4247^2 + 1980^2 .

It is a cyclic number.

It is not a de Polignac number, because 21957409 - 2⁷ = 21957281 is a prime.

It is not a weakly prime, because it can be changed into another prime (21957209) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10978704 + 10978705.

It is an arithmetic number, because the mean of its divisors is an integer number (10978705).

Almost surely, 2^21957409 is an apocalyptic number.

It is an amenable number.

21957409 is a deficient number, since it is larger than the sum of its proper divisors (1).

21957409 is an equidigital number, since it uses as much as digits as its factorization.

21957409 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 22680, while the sum is 37.

The square root of 21957409 is about 4685.8733444258. The cubic root of 21957409 is about 280.0229955604.

It can be divided in two parts, 21957 and 409, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21957409 in words is "twenty-one million, nine hundred fifty-seven thousand, four hundred nine".