Order | HCN n | prime factorization | prime exponents | prime factors | d(n) | primorial factorization |
---|---|---|---|---|---|---|
1 | 1 | 0 | 1 | |||
2 | 2 | 1 | 1 | 2 | ||
3 | 4 | 2 | 2 | 3 | ||
4 | 6 | 1,1 | 2 | 4 | ||
5 | 12 | 2,1 | 3 | 6 | ||
6 | 24 | 3,1 | 4 | 8 | ||
7 | 36 | 2,2 | 4 | 9 | ||
8 | 48 | 4,1 | 5 | 10 | ||
9 | 60 | 2,1,1 | 4 | 12 | ||
10 | 120 | 3,1,1 | 5 | 16 | ||
11 | 180 | 2,2,1 | 5 | 18 | ||
12 | 240 | 4,1,1 | 6 | 20 | ||
13 | 360 | 3,2,1 | 6 | 24 | ||
14 | 720 | 4,2,1 | 7 | 30 | ||
15 | 840 | 3,1,1,1 | 6 | 32 | ||
16 | 1260 | 2,2,1,1 | 6 | 36 | ||
17 | 1680 | 4,1,1,1 | 7 | 40 | ||
18 | 2520 | 3,2,1,1 | 7 | 48 | ||
19 | 5040 | 4,2,1,1 | 8 | 60 | ||
20 | 7560 | 3,3,1,1 | 8 | 64 | ||
21 | 10080 | 5,2,1,1 | 9 | 72 | ||
22 | 15120 | 4,3,1,1 | 9 | 80 | ||
23 | 20160 | 6,2,1,1 | 10 | 84 | ||
24 | 25200 | 4,2,2,1 | 9 | 90 | ||
25 | 27720 | 3,2,1,1,1 | 8 | 96 | ||
26 | 45360 | 4,4,1,1 | 10 | 100 | ||
27 | 50400 | 5,2,2,1 | 10 | 108 | ||
28 | 55440 | 4,2,1,1,1 | 9 | 120 | ||
29 | 83160 | 3,3,1,1,1 | 9 | 128 | ||
30 | 110880 | 5,2,1,1,1 | 10 | 144 | ||
31 | 166320 | 4,3,1,1,1 | 10 | 160 | ||
32 | 221760 | 6,2,1,1,1 | 11 | 168 | ||
33 | 277200 | 4,2,2,1,1 | 10 | 180 | ||
34 | 332640 | 5,3,1,1,1 | 11 | 192 | ||
35 | 498960 | 4,4,1,1,1 | 11 | 200 | ||
36 | 554400 | 5,2,2,1,1 | 11 | 216 | ||
37 | 665280 | 6,3,1,1,1 | 12 | 224 | ||
38 | 720720 | 4,2,1,1,1,1 | 10 | 240 |
Sunday, 1 April 2018
Highly Composite Numbers
Today I turned 25200 days old and I was surprised to find that this number has a staggering 347 entries in the Online Encyclopaedia of Integer Sequences (OEIS). Most numbers of this size are lucky to have more than a dozen entries. So what's so special about 25200? Well, it turns out to be a highly composite number, a term first coined by Ramanujan in 1915 and defined as a number that sets a record for the highest number of factors (in this case 90). Here is a table from Wikipedia showing details for the first 38 highly composite numbers (sequence A002182 in the OEIS).
All highly composite numbers are products of primorials as can be see from rightmost column of the table. In the case of 25200, the primorial factorisation is \( 2^2 \times 30 \times 210 \). There is a formula for calculating the number of factors for a number n:$$ \text{If }n=\prod_{i=1}^k p_i \, c^i \text{ then } d(n)=\prod_{i=1}^k (c^i+1)$$For example: $$ 25200=2^4\cdot 3^2\cdot 5^2\cdot 7 $$ $$ d(25200)=(4+1) \cdot (2+1) \cdot (2+1) \cdot (1+1) = 5 \cdot 3 \cdot 3 \cdot 2 = 90 $$The sequence of indices is non-increasing when the prime factor bases are placed in ascending order (4, 2, 2, 1 in the case of 25200). The final index is always 1 except in the cases of 4 and 36 where it is 2, thus making 1, 2 and 4 the only square, highly composite numbers.
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