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A Question to Trouble Even the Best of Youby adityab88
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#1
Apr507, 10:37 AM

P: 10

Find the smallest possible arithmetic sequence consisting of seven primes.
For example, the smallest possible arithmetic sequence consisting of five primes is: 5, 11, 17, 23, 29. By the way, by "smallest possible" I mean that the last term in the sequence must be the smallest possible of all such sequences. 


#3
Apr607, 11:44 PM

P: 367

Gib Z, you might want to think about that statement some more. It has been proven that there are arithmetic sequences of primes of any finite length.



#4
Apr707, 04:50 AM

HW Helper
P: 3,348

A Question to Trouble Even the Best of You
Shhh! lol
Ill reword it then, There is no such sequence that I can be bothered to find and that has any mathematical value. To find such a sequence all one needs is some programming knowledge and to be really bored. 


#5
Apr707, 05:00 AM

Sci Advisor
HW Helper
PF Gold
P: 12,016

What can readily be established is that any member of such a sequence must have the same last digit.
So there, I have narrowed it down immensely. 


#6
Apr707, 05:37 AM

P: 529

Yeah, is this a computing problem or a math problem?
(Actually, it sounds more like a Google problem) 


#8
Apr707, 11:55 AM

P: 14




#9
Apr707, 07:04 PM

HW Helper
P: 3,348

No idea don't ask me lol.
If you want, start a thread here inquiring about this. But not C++ I know. 


#10
Apr807, 12:19 AM

P: 14

Regarding adityab88's question, here is a solution I've just gotten: 1, 11, 31, 61, 101, 151, 211, 281 And it's the smallest possible because 361 is not a prime 


#11
Apr807, 01:02 AM

P: 1,075

ie. 111=10, whereas 3111=20, 6131=30 etc.. 


#12
Apr807, 02:08 AM

P: 14

[tex] X_n = X_{n1} + ( (n1) * 10 ) [/tex] Starting from (n = 1), [tex] X_0 = 1 [/tex] Is my solution too bad? 


#13
Apr807, 02:18 AM

P: 1,075

An arithmetic sequence has terms of the form: a_{n}=a_{0}+n*d Where a_{0} is the first term, and d is the common difference between terms. 


#14
Apr807, 03:05 AM

P: 10

hello ... ppl
i am quite sure you do not need programming equipment. just your brains will suffice; i am sure of this becuase i saw this question on a maths paper, where computers were not allowed this is all i have: the difference between the consecutive primes has to be a multiple of 30, i have shown this using some number theory. so, for example, the difference could be 60; a sequence like this (7, 67, 127, ...). good luck and have fun! 


#15
Apr807, 04:52 AM

P: 998

The spacing also can't be 30:
7, 37, ..., 187 doesn't work because 187 = 11*17. But for any prime [itex]p \neq 7[/itex], we have [itex] p \equiv 2k[/itex] (mod 7) for some k, [itex]1 \leq k \leq 6[/itex]. Since 30 mod 7 = 2 that means that p+30q is divisible by 7 for some q, 1<q<=6. That can be generalized to remove some other spacings too; I'm tired and will figure it out in the morning! Edit: OK, I lied about figuring it out later! You can also eliminate all other spacings, except those which are products of 7, in cases for any sequence which does not start with 7. So the sequence has to start with 7, or the spacing is a multiple of 7 (ie. 210, 420, etc.). A little bit of experimenting then yields the sequence: 7, 157, 307, 457, 607, 757, 907 


#17
Apr1207, 06:38 AM

P: 10

how long did it take you to "experiment" and find the final sequence? cause i was looking for it, too, but my experimentation did not really work. 


#18
Apr1207, 01:12 PM

P: 998

About five seconds . It makes sense to start at 7; if you can find a sequence starting with 7 that has spacing smaller than 210 (and there's no smaller sequence starting with 7), then that's obviously the smallest one.
Of course, from what I've done there was no particular reason to confine myself to sequences starting with 7, so it would be fair to consider it a lucky coincidence. 


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