21355951 has 4 divisors (see below), whose sum is σ = 21367504. Its totient is φ = 21344400.

The previous prime is 21355949. The next prime is 21355963. The reversal of 21355951 is 15955312.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a 4-Lehmer number, since φ(21355951) divides (21355951-1)⁴.

It is a cyclic number.

It is not a de Polignac number, because 21355951 - 2¹ = 21355949 is a prime.

It is a super-2 number, since 2×21355951² = 912153286228802, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21355921) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2310 + ... + 6931.

It is an arithmetic number, because the mean of its divisors is an integer number (5341876).

It is a Poulet number, since it divides 2^21355950-1.

Almost surely, 2^21355951 is an apocalyptic number.

21355951 is the 2311-st decagonal number.

21355951 is a deficient number, since it is larger than the sum of its proper divisors (11553).

21355951 is an equidigital number, since it uses as much as digits as its factorization.

21355951 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 11552.

The product of its digits is 6750, while the sum is 31.

The square root of 21355951 is about 4621.2499391398. The cubic root of 21355951 is about 277.4424889611.

The spelling of 21355951 in words is "twenty-one million, three hundred fifty-five thousand, nine hundred fifty-one".