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    <div class="moz-cite-prefix">Hi Mikhail and Scilabers<br>
    </div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">Thank you Mikhail for the equation with
      factorials, the URL to Wikipedia, and confirming my findings are
      OK (I really wasn't sure).</div>
    <div class="moz-cite-prefix">Although the specifics of my case (with
      codes on a lock) seems very constrained and 'special' - I suppose
      it is common after all.</div>
    <div class="moz-cite-prefix">It is interesting to see the math is
      quite simple.<br>
    </div>
    <div class="moz-cite-prefix">It is also interesting to see the
      'distribution' of combinations:</div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">
      <table width="873" cellspacing="0" cellpadding="0" border="0">
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            style="mso-width-source:userset;mso-width-alt:3840;width:79pt"
            width="105"> <col style="width:48pt" width="64" span="12">
        </colgroup><tbody>
          <tr style="height:15.0pt" height="20">
            <td style="height:15.0pt;width:79pt" width="105" height="20">Buttons
              pushed</td>
            <td style="width:48pt" width="64" align="right">0</td>
            <td style="width:48pt" width="64" align="right">1</td>
            <td style="width:48pt" width="64" align="right">2</td>
            <td style="width:48pt" width="64" align="right">3</td>
            <td style="width:48pt" width="64" align="right">4</td>
            <td style="width:48pt" width="64" align="right">5</td>
            <td style="width:48pt" width="64" align="right">6</td>
            <td style="width:48pt" width="64" align="right">7</td>
            <td style="width:48pt" width="64" align="right">8</td>
            <td style="width:48pt" width="64" align="right">9</td>
            <td style="width:48pt" width="64" align="right">10</td>
            <td style="width:48pt" width="64">Sum =</td>
          </tr>
          <tr style="height:15.0pt" height="20">
            <td style="height:15.0pt" height="20">Combinations</td>
            <td align="right">1</td>
            <td align="right">10</td>
            <td align="right">45</td>
            <td align="right">120</td>
            <td align="right">210</td>
            <td align="right">252</td>
            <td align="right">210</td>
            <td align="right">120</td>
            <td align="right">45</td>
            <td align="right">10</td>
            <td align="right">1</td>
            <td align="right">1024</td>
          </tr>
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    <div class="moz-cite-prefix"><br>
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    <div class="moz-cite-prefix">It is not that 'safe' of a lock because
      someone can decode it within an hour or so. If you take e.g. 5
      seconds for trying each combo, we're talking about 1,4 hours to
      try all of them and maybe you hit jackpot somewhere in the middle.</div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">It is not advisable to choose 0 or 1
      buttons for the code. Coding 5 random numbers to be pushed on the
      lock will give the highest number of combinations, but 4-6 buttons
      gives a suitable range of codes, 7 could also be OK. Although more
      buttons pushed gives fewer combinations, it's unlikely that e.g. a
      thief will start with a high number of buttons pushed (pure
      psychology).<br>
    </div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">From a decoding point of view, the most
      interesting part of the lock is that it isn't as simple as
      decoding a combination lock with 3 dials where you can easily keep
      track of how far you are. It becomes complicated and one kinda
      needs a look-up table to try all the combinations and it requires
      more concentration to go through all the combinations.<br>
    </div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">Cheers,</div>
    <div class="moz-cite-prefix">Claus<br>
    </div>
    <div class="moz-cite-prefix"><br>
    </div>
    <div class="moz-cite-prefix">On 15-04-2022 17:59, Mikhail Urusov
      wrote:<br>
    </div>
    <blockquote type="cite"
cite="mid:CAOKci2fsbA6Txi=Md6LwP2HODYBSG-6ApX29VV3auJYnvsenbA@mail.gmail.com">
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          <div dir="ltr">Dear Claus,</div>
          <div dir="ltr"><br>
          </div>
          <div dir="ltr">Just in case you have not got an answer yet:</div>
          <div dir="ltr"><br>
          </div>
          <div dir="ltr">> I can 'invent' a calculation, which could
            be : =10*9*8/(3*2*1) = 120 ... if this indeed shows the
            internal workings, I'd like to know why. Sorry my
            combinatorics is so bad ... I </div>
          <div dir="ltr"><br>
          </div>
          <div>This is right. In general, the formula for the number of
            k-combinations of n elements is</div>
          <div>n! / ( k! (n-k)! ).</div>
          <div>Alternatively, you can write this as</div>
          <div>n*(n-1)*....*(n-k+1) / (k!).</div>
          <div>(This is the way you have written it above for n = 10 and
            k = 3.)</div>
          <div>The explanation in general is quite similar to this case:</div>
          <div>
            <p>> Two buttons pushed. There's 10 * 9 / 2 = 45
              combinations. Each button can only be pushed once, so once
              you've selected the first button, there's only 9 left, but
              also we divide by two because the combination are doubled,
              I mean for example the combination 1-2 = 2-1 ... the lock
              doesn't know the difference. If you spread out the
              possibilities in a 2D plane, it's like ignoring the
              diagonal (like pushing the same button twice) and also we
              either ignore the upper or lower triangle. Makes sense?</p>
            <div>For example, for n=10 and k=4:</div>
            If you consider "arrangements", i.e., the combinations,
            where the order matters (e.g., 1-2-5-7 and 2-7-5-1 are
            different), then you have 10*9*8*7 such arrangements (for
            the first button you have 10 variants, for the second one 9
            variants, etc), but each "combination" (that is, where the
            order does not matter) is counted 4! (=1*2*3*4) times
            (number of permutations of 4 elements), so you need to
            divide: (10*9*8*7)/(4!).</div>
          <div>For more information, see <a
              href="https://en.wikipedia.org/wiki/Combination"
              target="_blank" moz-do-not-send="true"
              class="moz-txt-link-freetext">https://en.wikipedia.org/wiki/Combination</a></div>
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                  <div>
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                      <div><br>
                      </div>
                      <div>Best regards,</div>
                      <div>Mikhail</div>
                      <div><br>
                      </div>
                    </div>
                  </div>
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        <br>
        <div class="gmail_quote">
          <div dir="ltr" class="gmail_attr">Am Do., 14. Apr. 2022 um
            13:51 Uhr schrieb Claus Futtrup <<a
              href="mailto:cfuttrup@gmail.com" moz-do-not-send="true"
              class="moz-txt-link-freetext">cfuttrup@gmail.com</a>>:<br>
          </div>
          <blockquote class="gmail_quote" style="margin:0px 0px 0px
            0.8ex;border-left:1px solid
            rgb(204,204,204);padding-left:1ex">
            <div>
              <p>Dear Scilabers</p>
              <p>I hope you can help me out. My combinatorics is a bit
                rusty.</p>
              <p>So, the spouse has purchased a lock and I wondered how
                many combinations are available?</p>
              <p>The lock has 10 push buttons, they are numbered
                1-2-3-4-5-6-7-8-9-0.</p>
              <p>From a programming point of view, any of the numbers
                can be set on or off, meaning there are 2^10 = 1024
                combinations, as far as I can see.</p>
              <p>I wonder how they are distributed, and how many of the
                numbers I should activate in the lock to maximize the
                number of combinations?</p>
              <p>Let's see, we have:</p>
              <p>None (none of the buttons are activated), there's
                exactly 1 combination for this situation. The lock is
                delivered from the manufacturer in this state.</p>
              <p>All (all of the buttons are activated), there's exactly
                1 combination for this situation as well (no
                variability).</p>
              <p>One button pushed. There's obviously 10 possible
                combinations (push any one of the 10 buttons).</p>
              <p>Two buttons pushed. There's 10 * 9 / 2 = 45
                combinations. Each button can only be pushed once, so
                once you've selected the first button, there's only 9
                left, but also we divide by two because the combination
                are doubled, I mean for example the combination 1-2 =
                2-1 ... the lock doesn't know the difference. If you
                spread out the possibilities in a 2D plane, it's like
                ignoring the diagonal (like pushing the same button
                twice) and also we either ignore the upper or lower
                triangle. Makes sense?</p>
              <p>Here starts my trouble. Three buttons pushed. Instead
                of looking at a 2D plane, I guess you spread out in 3D.
                The diagonal line is more than that - we have several
                planes where two of the three numbers are the same (and
                which are not allowed).</p>
              <p>To help myself out, I've tried to write all
                combinations where one of the push buttons is number 1.
                We select all combinations with the second button being
                either 2-3-4 and so on, and how many combinations do we
                then have for the third option? See table below:</p>
              <p> </p>
              <table width="128" cellspacing="0" cellpadding="0"
                border="0">
                <colgroup><col style="width:48pt" width="64" span="2"> </colgroup><tbody>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt;width:48pt" width="64"
                      height="20">1-2-x</td>
                    <td style="width:48pt" width="64" align="right">8</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-3-x</td>
                    <td align="right">7</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-4-x</td>
                    <td align="right">6</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-5-x</td>
                    <td align="right">5</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-6-x</td>
                    <td align="right">4</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-7-x</td>
                    <td align="right">3</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-8-x</td>
                    <td align="right">2</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">1-9-0</td>
                    <td align="right">1</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20"><br>
                    </td>
                    <td align="right">36</td>
                  </tr>
                </tbody>
              </table>
              <p>We can then do the same for the first button = number
                2, and we get : 7 + 6 + 5 + 4 + 3 + 2 + 1 = 28
                combinations and so on. We get:<br>
              </p>
              <p> </p>
              <table width="128" cellspacing="0" cellpadding="0"
                border="0">
                <colgroup><col style="width:48pt" width="64" span="2"> </colgroup><tbody>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt;width:48pt" width="64"
                      height="20">1-x-y</td>
                    <td style="width:48pt" width="64" align="right">36</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">2-x-y</td>
                    <td align="right">28</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">3-x-y</td>
                    <td align="right">21</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">4-x-y</td>
                    <td align="right">15</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">5-x-y</td>
                    <td align="right">10</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">6-x-y</td>
                    <td align="right">6</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">7-x-y</td>
                    <td align="right">3</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20">8-9-0</td>
                    <td align="right">1</td>
                  </tr>
                  <tr style="height:15pt" height="20">
                    <td style="height:15pt" height="20"><br>
                    </td>
                    <td align="right">120</td>
                  </tr>
                </tbody>
              </table>
              <p>OK, so that was with three buttons pushed. It's always
                good to know the answer (if it's correct :-/ I hope it
                is), but it's a tedious process and I was wondering if
                you could point me to an easy calculation instead? ...
                Ideally something that expands to 4 and 5 buttons.</p>
              <p>I can 'invent' a calculation, which could be :
                =10*9*8/(3*2*1) = 120 ... if this indeed shows the
                internal workings, I'd like to know why. Sorry my
                combinatorics is so bad ... I haven't played in this
                field for a while.<br>
              </p>
              Best regards,
              <p>Claus<br>
              </p>
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