18 = 1234567 + 8 + 9
18 = 98 + 7 − 65 − 43 + 21
18 = 0^2 + 1^8 + 2^7 − 3^9 + 4^5 + 5^6 + 6^0 + 7^4 + 8^3 + 9^1
18 = 3^2 + 3^2
18 = 9 + 9 and 81 = 9 × 9
18 = 5+8+3+2 and 183 = 5832
18 = 4 × 4 + 4 −√4 = (44 ÷ √4) − 4
186 = 26+56+56+56+76+76+96+96+106+146+176
18 is a Harshad number ( Niven number), which is an integer divisible by the sum of its digits (1+8=9, and 18 is divisible by 9).
18 is divisible by 1, 2, 3, 6, and 9 and their sum is 21, thus 18 is an abundant number. Abundant number : 12, 18, 20, 24, 30, 36, 40, 42, 48, 54, 56, 60, 66, 70, 72, 78, 80, 84, 88, 90, 96, 100, 102, 104, 108, 112, 114, 120, ... (sequence A005101 in the OEIS).
18 = (1 + 1) × (11 − 1 − 1)
= 2(2+2) + 2
= 3 × (3 + 3)
= 4 × 4 + (4 + 4)/4
= 5 + (55 + 5 + 5)/5
= 6 + 6 + 6
= 7 + 77/7
= 8 + (88 − 8)/8
= 9 + 9
Number n which is the sum of 3 nonzero 4th powers
Number k such that (11*10^k + 19)/3 is prime
Numbers k such that 2^k + 9 is prime. (262153)
Number k such that 9^k + 2 is prime.
Number of squarefree graphs on 5 vertices
Number of ways to write 14 as an ordered sum of 4 nonprime numbers.
Number of points on surface of octahedron; also coordination sequence for cubic lattice: a(0) = 1; for n > 0, a(n) = 4n^2 + 2. n=2
Number of Integer partitions of 18: 385
a(n) = n^3 - 3*n , n = 2
Number of distinct products i*j*k for 1 <= i <= j < k <= n, n = 5
Number of ways to partition 2n+1 into distinct positive integers, n = 6
Representations, Binary to Hexadecimal:
10010_2
200_3
102_4
33_5
30_6
24_7
22_8
20_9
17_11
16_12
15_13
14_14
13_15
12_16
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