21212121 has 32 divisors (see below), whose sum is σ = 33331968. Its totient is φ = 11750400.

The previous prime is 21212119. The next prime is 21212131. The reversal of 21212121 is 12121212.

21212121 is an esthetic number in base 10, because in such base its adjacent digits differ by 1.

It is not a de Polignac number, because 21212121 - 2¹ = 21212119 is a prime.

It is an alternating number because its digits alternate between even and odd.

21212121 is an undulating number in base 10.

21212121 is a modest number, since divided by 212121 gives 21 as remainder.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21212097 and 21212106.

It is not an unprimeable number, because it can be changed into a prime (21212101) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 154765 + ... + 154901.

It is an arithmetic number, because the mean of its divisors is an integer number (1041624).

Almost surely, 2^21212121 is an apocalyptic number.

21212121 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21212121 is a deficient number, since it is larger than the sum of its proper divisors (12119847).

21212121 is a wasteful number, since it uses less digits than its factorization.

21212121 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 321.

The product of its digits is 16, while the sum is 12.

The square root of 21212121 is about 4605.6618416901. The cubic root of 21212121 is about 276.8182371361.

Multiplying 21212121 by its sum of digits (12), we get a palindrome (254545452).

Adding to 21212121 its reverse (12121212), we get a palindrome (33333333).

Subtracting from 21212121 its reverse (12121212), we obtain a palindrome (9090909).

It can be divided in two parts, 21212 and 121, that multiplied together give a palindrome (2566652).

The spelling of 21212121 in words is "twenty-one million, two hundred twelve thousand, one hundred twenty-one".