# How many combinations will be generated if below conditions are put?

I need a generalized formula for a set having size(s) having below restrictions,

for ex. X = 2,7,11,17,26

I want only the first combination 2+7 & ignore all of the combinations that start from 2+? (2-num combinations only) same for 3-num consider 2+7+11 & ignore all 2+7+?

Formula should be applicable for any set X without size restriction.

• Please clarify your question a bit. You write combinations but also write sums (2+7) but I assume you want the number of selections of 2 numbers drawn from $X = \{1, 7, 11, 17, 26\}$ without repetition and ignoring orderings (that's what combination usually means)? – ttnick Jun 24 at 8:14
• for me sum of 2+7=9 or any combination is not important. I am interested in knowing what if we consider only the first combination & ignore all of the combinations in that particular branch then what will be the formula to know all of the first level combinations count. – Bhagwan Parge Jun 24 at 8:38
• The question is very unclear and it is hard to figure out what you want to achieve. – Navjot Waraich Jun 24 at 9:13
• @NavjotWaraich some information & image included for more clarity, hope it helps. – Bhagwan Parge Jun 24 at 11:04

$$2^{n-1}-1$$

Hint: 1) if you are summing with $$k$$ numbers $$x_1,x_2,.., x_k$$ note that once you choose upto $$k-1$$, last selection is forced on you as the next ascending number to $$x_{k-1}$$. 2) You are allowed to choose first $$k-1$$ numbers from n-1 numbers

First, let's count the number of 2-combinations. You already mentioned one combination: $$(2,7)$$, and then there are $${4 \choose 2}$$ combinations for the remaining set $$\{7, 11, 17, 26\}$$:

$$S_2 = 1 + {4 \choose 2}$$

With 3-combinations you said you should include $$(2, 7, 11)$$. Then, there are $${4 \choose 3}$$ combinations for choosing triplets from $$\{7, 11, 17, 26\}$$ and $${4 \choose 3}$$ combinations for choosing triplets from $$\{2, 11, 17, 26\}$$.

But with the last two numbers, we have counted some stuff twice. Concretely, we need to subtract the number of 3-combinations made from $$\{11, 17, 26\}$$, which is $${3 \choose 3} = 1$$.

$$S_3 = 1 + {4 \choose 3} + {4 \choose 3} - 1 = 2 {4 \choose 3}$$

In the end, you have $$S = 1 + {4 \choose 2} + 2 {4 \choose 3} = 1 + 6 + 8 = 15$$ combinations.

• Thanks. Can you give a generalized formula (set with size n) – Bhagwan Parge Jun 24 at 11:03