See here: How can I prove the last digit of $(2^{121985292}-1)$ is $5$

The most upvoted solution, which has 13 upvotes, is nice, short, and intuitive, but not rigorous. The best rigorous solution has 3 upvotes while mine, which is more complete has 1.

Not really whining, but something seems wrong with the upvote system.

EDIT: He said: 2^1 ends in 2 2^2 ends in 4 2^3 ends in 8 2^4 ends in 6 2^5 ends in 2 2^4k ends in 6.

Why does 2^4k end in 6? It is not explained at all.

  • 14
    $\begingroup$ Welcome to SE! This is how it works. :-) $\endgroup$
    – quid Mod
    Dec 14 '14 at 2:12
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    $\begingroup$ I don't think Fermat's Little Theorem is required to tell that $2^4$ is congruent to $1$ modulo $5$, or that $6 \times 6 = 36$ which also ends in $6$. There is rigor, and then there is unneeded rigor... $\endgroup$ Dec 14 '14 at 2:16
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    $\begingroup$ His answer was more or less a framework for an inductive proof so it's not that bad, in all honesty. The number of upvotes an answer receives is not a reflection of completeness or the beauty of an answer. I've had plenty of very complete and beautiful answers which didn't receive much attention whereas simple answers received a lot of attention. It sucks when an answer you worked hard on doesn't get a lot of attention but after a while, you let go of such notions. The answers are for the OP to learn from, not for you to gain fame from. $\endgroup$ Dec 14 '14 at 2:33
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    $\begingroup$ I didn't vote on that question, but if I had I would have voted for that answer. That answer computed until there was a repeat. This is the basis for a simple yet completely rigorous way to do it. These sorts of problems can be explained to grade school kids. (But yes, in general, voting can be arbitrary; that is its nature.) $\endgroup$
    – aes
    Dec 14 '14 at 2:51
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    $\begingroup$ The upvote button says "This answer is useful" not "This answer is rigorous". $\endgroup$ Dec 14 '14 at 3:22
  • $\begingroup$ This is off-topic but what about this ? $\endgroup$ Dec 14 '14 at 4:10
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    $\begingroup$ I like the answer. It is crystal clear, easily understood by almost anyone, and a straightforward induction argument makes it perfectly rigorous, as noted above. $\endgroup$
    – user169852
    Dec 14 '14 at 4:18
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    $\begingroup$ If you want to discuss voting on this specific question, you should tag this as (sspecific-question) (see the tag-info). If you want to discuss general issue and this particular question only serves as an example, you should probably explain that in the post. $\endgroup$ Dec 14 '14 at 6:03

Welcome to Math.SE! The StackExchange platform is (arguably) the best question and answer service online, but it does have issues; one of the most noticeable is that people do not always vote the way we think they should.

The best response you can have is to: 1) grow a thick skin, 2) answer questions for reasons other than reputation points, and 3) do your part by voting early and often.

An aside: with a username that contains the term "troll," you may find that people aren't too sympathetic when you complain. :)


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