23; its only positive divisors are 1 and 23

16; it has the form n²

6; its proper positive divisors are 1, 2, and 3, which sum up to 6

12; all the numbers from 1 to 11 can be expressed as sums of its divisors 1, 2, 3, 4, and 6

15; it is the product of two primes

84; it is divisible by sum of its digits

1111; it only contains 1 as the digits

45; 45²=2025 and 20+25=45

15; it has the form 2ⁿ-1

6; its positive divisors are 1, 2, 3, and 6, whose average is an integer

12; its proper positive divisors are 1, 2, 3, 4, and 6, whose sum 16 is larger than 12

133; if you write down the sum of the square of the digits, it eventually reaches 1 (133, 19, 82, 68, 100, 1)

161; it is the same when read backwards

17; it has the form 2ⁿ+1 where n=4 is a power of 2

49; it is divisible by 7, which is neither 1 nor 49

12; it can be expressed as the sum of its distinct proper divisors

48; its prime divisors are 2, 3, or 5

15; it can be expressed as the sum of two or more consecutive integers

36; for every prime divisor p of 36, p² is a divisor of 36

27; it has the form n³

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