21306506 has 20 divisors (see below), whose sum is σ = 34715802. Its totient is φ = 9807408.

The previous prime is 21306479. The next prime is 21306539. The reversal of 21306506 is 60560312.

21306506 = T₁₈₀₄ + T₁₈₀₅ + ... + T₁₈₁₆.

It can be written as a sum of positive squares in 5 ways, for example, as 2739025 + 18567481 = 1655^2 + 4309^2 .

It is an unprimeable number.

It is a polite number, since it can be written in 9 ways as a sum of consecutive naturals, for example, 56936 + ... + 57308.

Almost surely, 2^21306506 is an apocalyptic number.

21306506 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

21306506 is a deficient number, since it is larger than the sum of its proper divisors (13409296).

21306506 is an frugal number, since it uses more digits than its factorization.

21306506 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 427 (or 388 counting only the distinct ones).

The product of its (nonzero) digits is 1080, while the sum is 23.

The square root of 21306506 is about 4615.8970959067. The cubic root of 21306506 is about 277.2282045115.

Adding to 21306506 its reverse (60560312), we get a palindrome (81866818).

It can be divided in two parts, 21306 and 506, that added together give a palindrome (21812).

The spelling of 21306506 in words is "twenty-one million, three hundred six thousand, five hundred six".