1643821 has 2 divisors, whose sum is σ = 1643822. Its totient is φ = 1643820.

The previous prime is 1643819. The next prime is 1643827. The reversal of 1643821 is 1283461.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 902500 + 741321 = 950^2 + 861^2 .

It is a cyclic number.

It is not a de Polignac number, because 1643821 - 2¹ = 1643819 is a prime.

Together with 1643819, it forms a pair of twin primes.

It is a d-powerful number, because it can be written as 1 + 6³ + 4⁷ + 3¹³ + 8⁵ + 2⁷ + 1 .

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1643827) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 821910 + 821911.

It is an arithmetic number, because the mean of its divisors is an integer number (821911).

2^1643821 is an apocalyptic number.

It is an amenable number.

1643821 is a deficient number, since it is larger than the sum of its proper divisors (1).

1643821 is an equidigital number, since it uses as much as digits as its factorization.

1643821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1152, while the sum is 25.

The square root of 1643821 is about 1282.1158294008. The cubic root of 1643821 is about 118.0188852279.

It can be divided in two parts, 1643 and 821, that multiplied together give a triangular number (1348903 = T₁₆₄₂).

The spelling of 1643821 in words is "one million, six hundred forty-three thousand, eight hundred twenty-one".