Find the number of ordered pairs of integers (a,b) such that the sequence 3,4,5,a,b,30,40,50 is strictly increasing and no set of four (not necessarily consecutive) terms forms an arithmetic progression.
ChatGPT said:
To solve this problem, we need to find all possible ordered pairs (a,b)(a, b)(a,b) such that the sequence
3,4,5,a,b,30,40,503, 4, 5, a, b, 30, 40, 503,4,5,a,b,30,40,50
satisfies two conditions:
- The sequence is strictly increasing.
- No subset of four terms (not necessarily consecutive) forms an arithmetic progression.
Step 1: Strictly Increasing Condition
Since the sequence is strictly increasing, we must have:
5<a<b<30.5 < a < b < 30.5<a<b<30.
Thus, aaa and bbb must be integers satisfying:
a∈{6,7,…,29},b∈{a+1,a+2,…,29}.a \in \{6, 7, \ldots, 29\}, \quad b \in \{a+1, a+2, \ldots, 29\}.a∈{6,7,…,29},b∈{a+1,a+2,…,29}.
This gives b−ab - ab−a possible values for bbb for each valid aaa. We'll revisit this after considering the second condition.
Step 2: No Arithmetic Progression of Four Terms
An arithmetic progression of four terms satisfies:
x1,x2,x3,x4withx2−x1=x3−x2=x4−x3.x_1, x_2, x_3, x_4 \quad \text{with} \quad x_2 - x_1 = x_3 - x_2 = x_4 - x_3.x1,x2,x3,x4withx2−x1=x3−x2=x4−x3.
We need to ensure that no such progression exists for any subset of {3,4,5,a,b,30,40,50}\{3, 4, 5, a, b, 30, 40, 50\}{3,4,5,a,b,30,40,50}. The fixed terms 3,4,5,30,40,503, 4, 5, 30, 40, 503,4,5,30,40,50 cannot form an arithmetic progression of four terms themselves. So, we need to carefully place aaa and bbb to avoid creating progressions.
Step 2.1: Common Differences in the Fixed Terms
We analyze the fixed terms {3,4,5,30,40,50}\{3, 4, 5, 30, 40, 50\}{3,4,5,30,40,50} and their possible differences:
- Differences between consecutive terms: 4−3=14 - 3 = 14−3=1, 5−4=15 - 4 = 15−4=1, 30−5=2530 - 5 = 2530−5=25, 40−30=1040 - 30 = 1040−30=10, 50−40=1050 - 40 = 1050−40=10.
- We must ensure aaa and bbb do not create a four-term arithmetic progression involving any subset of these terms.
Step 2.2: Constraints on
- aaa cannot be placed such that it creates an arithmetic progression with 3,4,53, 4, 53,4,5 or 30,40,5030, 40, 5030,40,50.
- For example:
- If a=15a = 15a=15, then {3,9,15,21}\{3, 9, 15, 21\}{3,9,15,21} forms an arithmetic progression, which is not allowed.
- Similarly, bbb must avoid creating progressions when paired with any three other terms.
Step 3: Systematic Counting of Valid Pairs
To solve this rigorously, we would need to:
- Identify all subsets of size 4 from {3,4,5,a,b,30,40,50}\{3, 4, 5, a, b, 30, 40, 50\}{3,4,5,a,b,30,40,50}.
- For each subset, check whether it forms an arithmetic progression.
- Exclude all (a,b)(a, b)(a,b) that lead to any arithmetic progression.
This process is complex and computationally intensive. However, if you'd like, I can implement a programmatic solution to enumerate and verify all pairs (a,b)(a, b)(a,b). Would you like me to proceed?
You said:
Yes
ChatGPT said:
There are 228 valid ordered pairs (a,b)(a, b)(a,b) such that the sequence 3,4,5,a,b,30,40,503, 4, 5, a, b, 30, 40, 503,4,5,a,b,30,40,50 is strictly increasing and no subset of four terms forms an arithmetic progression.
Here are the first 10 valid pairs for verification:
(7, 8), (7, 10), (7, 11), (7, 12), (7, 13), (7, 14), (7, 15), (7, 16), (7, 17), (7, 18)