28, Perfect Number, can be divided (1, 2, 4, 7, 14) and 1 + 2 + 4 + 7 + 14 = 28
28 = 13 + 33
28 = 12 + 3 − 4 − 5 − 67 + 89
28 = 98 − 7 + 6 − 5 − 43 − 21
28 = −0^0 + 1^9 + 2^6 + 3^7 − 4^8 + 5^3 + 6^2 + 7^1 + 8^4 + 9^5
28 = (1 + 1 + 1)(1+1+1) + 1
= 22 + 2 + 2 + 2
= 33 + 3/3
= 44 − 4 × 4= (4 + 4)×4 − 4 = 4!+ 4 + 4 - 4
= 5 × 5 + 5 − (5 + 5)/5
= 6 + (66 + 66)/6
= 7 × (77/7 − 7)
= 8 + 8 + (88 + 8)/8
= 9 + 9 + 9 + 9/9
28 = n * Prime(n) = 4*Prime(4)
Number of ways to write 15 as an ordered sum of 4 nonprime numbers.
Numbers k such that k^4 + 1 is prime.
Sum of the first n primes
Keith number or Repfigit (Repetitive Fibonacci-like digit)
28 cannot be written as a sum of 3 squares. (Integers that are not a sum of three squares)
Representations, Binary to Hexadecimal:
11100_2
1001_3
130_4
103_5
44_6
40_7
34_8
31_9
26_11
24_12
22_13
20_14
1d_15
1c_16
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Undergraduate Texts in Mathematics
Graduate Studies in Mathematics
