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).
OrderHCN
n
prime
factorization
prime
exponents
prime
factors
d(n)primorial
factorization
1101
22112
34223
461,124
5122,136
6243,148
7362,249
8484,1510
9602,1,1412
101203,1,1516
111802,2,1518
122404,1,1620
133603,2,1624
147204,2,1730
158403,1,1,1632
1612602,2,1,1636
1716804,1,1,1740
1825203,2,1,1748
1950404,2,1,1860
2075603,3,1,1864
21100805,2,1,1972
22151204,3,1,1980
23201606,2,1,11084
24252004,2,2,1990
25277203,2,1,1,1896
26453604,4,1,110100
27504005,2,2,110108
28554404,2,1,1,19120
29831603,3,1,1,19128
301108805,2,1,1,110144
311663204,3,1,1,110160
322217606,2,1,1,111168
332772004,2,2,1,110180
343326405,3,1,1,111192
354989604,4,1,1,111200
365544005,2,2,1,111216
376652806,3,1,1,112224
387207204,2,1,1,1,110240
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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