Talk:Special factorials: Difference between revisions
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: I think it's just that the formula literally produces 0 for n = 0. |
: I think it's just that the formula literally produces 0 for n = 0. |
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: First run through, i is 1, this produces -1. Second run through, i is 0, this produces 1 and their sum is 0. --[[User:Chunes|Chunes]] ([[User talk:Chunes|talk]]) 17:36, 16 March 2021 (UTC) |
: First run through, i is 1, this produces -1. Second run through, i is 0, this produces 1 and their sum is 0. --[[User:Chunes|Chunes]] ([[User talk:Chunes|talk]]) 17:36, 16 March 2021 (UTC) |
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So the math summation notation goes backwards automatically when i > n? This may make a good task -- C will do one iteration and stop, etc. --[[User:Wherrera|Wherrera]] ([[User talk:Wherrera|talk]]) 21:11, 16 March 2021 (UTC) |
Revision as of 21:11, 16 March 2021
Reverse factorial algorithm
I took a stab at translating the reverse factorial algorithm used in the Factor entry to Java. It should be almost as efficient as taking the factorial itself. <lang java>public static int rf(int n) {
if (n == 1) return 0; //1 has two answers -- return the lower one
int a = 1; int b = 1; while (n > a) { b++; a = a * b; } if (a == n) return b; else return -1; //undefined }</lang>
--Chunes (talk) 17:06, 16 March 2021 (UTC)
- Note that the factorial inverse (or reverse factorial) of unity has two possible answers: zero and unity.
- It is normal when searching a series (in this case, the series of factorial products) to use the first match found in the series. -- Gerard Schildberger (talk) 17:37, 16 March 2021 (UTC)
Why is af(0) 0?
Is it that 0 is the additive identity the way that factorial of 0 is 1 is the multiplicative identity? If so why should it be the identity anyway?
- I think it's just that the formula literally produces 0 for n = 0.
- First run through, i is 1, this produces -1. Second run through, i is 0, this produces 1 and their sum is 0. --Chunes (talk) 17:36, 16 March 2021 (UTC)
So the math summation notation goes backwards automatically when i > n? This may make a good task -- C will do one iteration and stop, etc. --Wherrera (talk) 21:11, 16 March 2021 (UTC)