20721553 has 2 divisors, whose sum is σ = 20721554. Its totient is φ = 20721552.

The previous prime is 20721541. The next prime is 20721583. The reversal of 20721553 is 35512702.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20684304 + 37249 = 4548^2 + 193^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20721553 is a prime.

It is not a weakly prime, because it can be changed into another prime (20721583) 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, 10360776 + 10360777.

It is an arithmetic number, because the mean of its divisors is an integer number (10360777).

Almost surely, 2^20721553 is an apocalyptic number.

It is an amenable number.

20721553 is a deficient number, since it is larger than the sum of its proper divisors (1).

20721553 is an equidigital number, since it uses as much as digits as its factorization.

20721553 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2100, while the sum is 25.

The square root of 20721553 is about 4552.0932547565. The cubic root of 20721553 is about 274.6676005862.

The spelling of 20721553 in words is "twenty million, seven hundred twenty-one thousand, five hundred fifty-three".