21151721 has 2 divisors, whose sum is σ = 21151722. Its totient is φ = 21151720.

The previous prime is 21151687. The next prime is 21151733. The reversal of 21151721 is 12715112.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18705625 + 2446096 = 4325^2 + 1564^2 .

It is a cyclic number.

It is not a de Polignac number, because 21151721 - 2¹⁴ = 21135337 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21151693 and 21151702.

It is not a weakly prime, because it can be changed into another prime (21051721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10575860 + 10575861.

It is an arithmetic number, because the mean of its divisors is an integer number (10575861).

Almost surely, 2^21151721 is an apocalyptic number.

It is an amenable number.

21151721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21151721 is an equidigital number, since it uses as much as digits as its factorization.

21151721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 140, while the sum is 20.

The square root of 21151721 is about 4599.1000206562. The cubic root of 21151721 is about 276.5552473079. Note that the first 3 decimals are identical.

Adding to 21151721 its reverse (12715112), we get a palindrome (33866833).

The spelling of 21151721 in words is "twenty-one million, one hundred fifty-one thousand, seven hundred twenty-one".