# 1 + 2 + 3 + 4 + ⋯

For example, this group <1,2,3,4,5> would be analysed like this:

1-2, 1-3, 1-4, 1-52-3, 2-4, 2-53-4, 3-54-5By creating little diagrams like this I"ve figured out the pattern which is as follows:

Users - Comparisons2 - 13 - 3 (+2)4 - 6 (+3)5 - 10 (+4)6 - 15 (+5)7 - 21 (+6)8 - 28 (+7)9 - 36 (+8)I need to lớn be able lớn take any number of users, và calculate how many comparisons it will take to lớn compare every user with every other user.

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Can someone please tell me what the formula for this is?

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edited May 2, 2014 at 15:23

MJD

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asked May 2, 2014 at 15:21

OwenOwen

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## 7 Answers 7

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You want to lớn know how many ways there are to choose $2$ users froma mix of $n$ users.

Generally, the number of ways lớn choose $k$ elements from a phối oforder $n$ (that is, all elements in the set are distinct) is denotedby $$inomnk$$

and is equivalent to $$fracn!(n-k)!k!$$

In the case of $k=2$ the latter equals lớn $$fracn!(n-2)!2!=fracn(n-1)2$$

which is also the sum of $1+2+...+n-1$.

For more information see Binomial coefficient & Arithmetic progression

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answered May 2, 2014 at 15:28

BelgiBelgi

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The sum of $0+cdots + n-1$ is $$frac12(n-1)n.$$

Here $n$ is the number of users; there are 0 comparisons needed for the first user alone, 1 for the second user (to compare them to lớn the first), 2 for the third user, and so on, up to the $n$th user who must be compared with the $n-1$ previous users.

For example, for $9$ people you are adding up $0+1+2+3+4+5+6+7+8$, which is equal to lớn $$frac12cdot 8cdot 9= frac722 = 36$$ & for $10$ people you may compute $$frac12cdot9cdot10 = frac902 = 45.$$

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answered May 2, năm trước at 15:25

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The following way to lớn getting the solution is beautiful and said lớn have been found by young Gauss in school. The idea is that the order of adding $1+2+cdots+n=S_n$ does not change the value of the sum. Therefore:

$$1 + 2 + ldots + (n-1) + n=S_n$$$$n + (n-1) + ldots + 2 + 1=S_n$$

Adding the two equations term by term gives

$$(n+1)+(n+1)+ldots+(n+1)=2S_n$$

so $n(n+1)=2S_n$. For $n$ persons, there are $S_n-1$ possibilities, as others answers have shown already nicely.

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answered May 2, năm trước at 15:33

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The discrete sum up to a finite value $N$ is given by,

$$sum_n=1^N n = frac12N(N+1)$$

**Proof:**

The proof by induction roughly boils down to:

$$S_N = 1+ 2 +dots+N$$

$$S_N+1= 1+ 2 + dots + N + (N+1) = underbracefrac12N(N+1)_S_N + (N+1)$$

assuming that the induction hypothesis is true. The right hand side:

$$fracN(N+1)2+(N+1)=frac(N+1)(N+2)2$$

which is precisely the induction hypothesis applied khổng lồ $S_N+1$.

Just for your own curiosity, the case $N=infty$ is of course divergent. However, with the use of the zeta function, it may be regularized lớn yield,

$$sum_n=1^inftyn = zeta(-1)=-frac112$$

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answered May 2, 2014 at 15:31

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$$N=2: 1 + 2 = (1 + 2) = 1 imes3$$

$$N=4: 1 + 2 + 3 + 4 = (1 + 4) + (2 + 3) = 2 imes5$$

$$N=6: 1 + 2 + 3 + 4 + 5 + 6 = (1 + 6) + (2 + 5) + (3 + 4) = 3 imes7$$

$$N=8: 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8= (1 + 8) + (2 + 7) + (3 + 6)+ (4 + 5) = 4 imes9$$

More generally, $N/2 imes(N + 1)$.

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For odd $N$, sum the $N-1$ first terms (using the even formula) together with $N$, giving $(N-1)/2 imes N+N=N imes(N+1)/2$.

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edited May 2, năm trước at 15:47

answered May 2, 2014 at 15:41

user65203user65203

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corsiKa: the formula explicitly shows the sum from 1 khổng lồ $N$ inclusive (triangular numbers) & is perfectly correct. It is NOT the formula for the number of comparisons between $N$ users. $endgroup$

–user65203

May 2, 2014 at 21:38

$egingroup$ The sum must be taken from $1$ lớn $Users-1$ inclusive. $endgroup$

–user65203

May 2, 2014 at 21:51

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Here is another way to find the sum of the first $n$ squares that generalizes to sums of higher powers.

$(k+1)^2 - k^2 = 2k+1$

$sum_k=1^n ( (k+1)^2 - k^2 ) = sum_k=1^n (2k+1)$

$(n+1)^2 - 1^2 = 2 sum_k=1^n k + n$

$sum_k=1^n k = frac (n+1)^2 - 1 - n 2 = fracn^2+n2$

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answered May 3, 2014 at 7:11

user21820user21820

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This is kind of a pseudo code:

Say you have n number of people, và you labeled them.

for i in (1,2,3,...,n), person i need to compare with all the people who has a number larger (strictly), so person i need to lớn compare (n-i) times.

so adding up would be(n-1) + (n-2) + ... + 3 + 2 + 1...

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which would be the sum from 1 khổng lồ (n-1)

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answered May 2, 2014 at 15:26

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