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Prime Sequences

Age 16 to 18
Challenge Level Yellow star
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In 2004 an exciting new result was proved in Number Theory by two young mathematicians Ben Green and Terence Tao. They proved that if you look in a long enough list of the prime numbers then you will be able to find numbers which form an arithmetic progression containing as many numbers as you choose! In this question we explore some of the interesting issues surrounding arithmetic progressions of prime numbers.


An $AP-k$ sequence is $k\geq 3$ primes in arithmetic progression. See examples
A simple arithmetic progression of three primes starts at $3$ with common difference $4$, giving rise to the progression of prime numbers
$$
3, 7,11
$$
This is an example of $AP-3$. Note that the sequence stops here because $11+4=15$, which is not a prime number. Another short arithmetic progression starts at $7$ with common difference $6$
$$
7, 13, 19
$$


This problem involves several linked parts leading up to a final challenge. Try some of the earlier questions to gain insights into the final challenge. These can be attempted in any order. You might find that you naturally ask yourself questions which are found later in the list of questions and you might find that one part helps in the consideration of another part. Of course, you are welcome to go straight to the final challenge. However, you might also wish to start with one of the earlier challenges and see how many of the other challenges you naturally discover whilst exploring the underlying mathematical structure.

Consider some of these three questions first:
Question A
Can you find an arithmetic progression of four primes?
Question B
How many prime APs beginning with $2$ can you find?
Question C
How many other arithmetic progressions of prime numbers from the list of primes below can you find?
Next consider some of these three questions:
Question A
Why is $3, 5, 7$ the only prime AP with common difference $2$?
Question B
What is the maximum length of a prime AP with common difference of $6$?
Question C
If the common difference of a prime AP is $N$ then the maximum length of the prime AP is $N-1$.
Now consider some of these three questions:
Question A
What is the maxiumum length of a prime AP with common difference $10$?
Question B
What is the max length of a prime AP with common difference $100, 1000, 10000$ ?
Question C
What are the possible lengths of prime APs with common difference $2p$, where $p$ is prime? Consider $p=3$ and $p> 3$ separately.

When you have thought about some of the previous problems you might like to try the final challenge


Prove that if an AP-$k$ does not begin with the prime $k$, then the common difference is a multiple of the primorial $k$#$ = 2\cdot 3\cdot 5 \cdot \dots \cdot j$, where $j$ is the largest prime not greater than $k$.

Once you have solved this, why not try to think of some other questions about prime APs to ask?


In doing these problems you might like to see this list of primes

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71  
73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173  
179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281  
283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383 389 397 401 409  
419 421 431 433 439 443 449 457 461 463 467 479 487 491 499 503 509 521 523 541  
547 557 563 569 571 577 587 593 599 601 607 613 617 619 631 641 643 647 653 659  
661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769 773 787 797 809  
811 821 823 827 829 839 853 857 859 863 877 881 883 887 907 911 919 929 937 941  
947 953 967 971 977 983 991 997 1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 1063 1069


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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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