1621721 has 2 divisors, whose sum is σ = 1621722. Its totient is φ = 1621720.

The previous prime is 1621717. The next prime is 1621723. The reversal of 1621721 is 1271261.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1597696 + 24025 = 1264^2 + 155^2 .

It is a cyclic number.

It is not a de Polignac number, because 1621721 - 2² = 1621717 is a prime.

Together with 1621723, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p₁₂₂₆₆₃ and since 1621721 and 122663 have the same sum of digits, it is a Honaker prime.

It is a junction number, because it is equal to n+sod(n) for n = 1621693 and 1621702.

It is not a weakly prime, because it can be changed into another prime (1621723) 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, 810860 + 810861.

It is an arithmetic number, because the mean of its divisors is an integer number (810861).

2^1621721 is an apocalyptic number.

It is an amenable number.

1621721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1621721 is an equidigital number, since it uses as much as digits as its factorization.

1621721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 168, while the sum is 20.

The square root of 1621721 is about 1273.4680993256. The cubic root of 1621721 is about 117.4876039435.

Adding to 1621721 its reverse (1271261), we get a palindrome (2892982).

The spelling of 1621721 in words is "one million, six hundred twenty-one thousand, seven hundred twenty-one".