21138421 has 2 divisors, whose sum is σ = 21138422. Its totient is φ = 21138420.

The previous prime is 21138409. The next prime is 21138427. The reversal of 21138421 is 12483112.

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., 13242321 + 7896100 = 3639^2 + 2810^2 .

It is a cyclic number.

It is not a de Polignac number, because 21138421 - 2¹⁹ = 20614133 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21138392 and 21138401.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21138427) 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, 10569210 + 10569211.

It is an arithmetic number, because the mean of its divisors is an integer number (10569211).

Almost surely, 2^21138421 is an apocalyptic number.

It is an amenable number.

21138421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21138421 is an equidigital number, since it uses as much as digits as its factorization.

21138421 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 384, while the sum is 22.

The square root of 21138421 is about 4597.6538582194. The cubic root of 21138421 is about 276.4972700556.

The spelling of 21138421 in words is "twenty-one million, one hundred thirty-eight thousand, four hundred twenty-one".