Theorem: Factors of Mersenne Number M67

The Mersenne number $M_{67}$ has the factors:
:$193 \, 707 \, 721$
:$761 \, 838 \, 257 \, 287$

Proof:
First we calculate $M_{67}$, which is $2^{67} - 1$:
:$2^2 = 2 \times 2 = 4$
{{begin-eqn}}
{{eqn | l = 2^4
      | r = 2^2 \times 2^2
      | c = 
}}
{{eqn | r = 4 \times 4
      | c = 
}}
{{eqn | r = 16
      | c = 
}}
{{end-eqn}}
{{begin-eqn}}
{{eqn | l = 2^8
      | r = 2^4 \times 2^4
      | c = 
}}
{{eqn | r = 16 \times 16
      | c = 
}}
{{eqn | r = 256
      | c = 
}}
{{end-eqn}}
{{begin-eqn}}
{{eqn | l = 2^{16}
      | r = 2^8 \times 2^8
      | c = 
}}
{{eqn | r = 256 \times 256
      | c = 
}}
{{eqn | r = 65 \, 536
      | c = 
}}
{{end-eqn}}
as follows:
<pre>
   256
 x 256
------
 1 536
12 800
51 200
------
65 536
------
</pre>
{{begin-eqn}}
{{eqn | l = 2^{32}
      | r = 2^{16} \times 2^{16}
      | c = 
}}
{{eqn | r = 65 \, 536 \times 65 \, 536
      | c = 
}}
{{eqn | r = 4 \, 294 \, 967 \, 296
      | c = 
}}
{{end-eqn}}
as follows:
<pre>
       65 536
     x 65 536
      -------
      393 216
    1 966 080
   32 768 000
  327 680 000
3 932 160 000
-------------
4 294 967 296
-------------
</pre>
{{begin-eqn}}
{{eqn | l = 2^{64}
      | r = 2^{32} \times 2^{32}
      | c = 
}}
{{eqn | r = 4 \, 294 \, 967 \, 296 \times 4 \, 294 \, 967 \, 296
      | c = 
}}
{{eqn | r = 18 \, 446 \, 744 \, 073 \, 709 \, 551 \, 616
      | c = 
}}
{{end-eqn}}
as follows:
<pre>
             4 294 967 296
           x 4 294 967 296
            --------------
            25 769 803 776
           386 547 056 640
           858 993 459 200
        30 064 771 072 000
       257 698 037 760 000
     3 865 470 566 400 000
    17 179 869 184 000 000
   386 547 056 640 000 000
   858 993 459 200 000 000
17 179 869 184 000 000 000
--------------------------
18 446 744 073 709 551 616
--------------------------
 1 233 444 664 532 221 1
</pre>
{{begin-eqn}}
{{eqn | l = 2^{67}
      | r = 2^{64} \times 2^3
      | c = 
}}
{{eqn | r = 18 \, 446 \, 744 \, 073 \, 709 \, 551 \, 616 \times 8
      | c = 
}}
{{eqn | r = 147 \, 573 \, 952 \, 589 \, 676 \, 412 \, 928
      | c = 
}}
{{end-eqn}}
as follows:
<pre>
 18 446 744 073 709 551 616
x                         8
---------------------------
147 573 952 589 676 412 928
---------------------------
 63 355 33  525  74 414 14
</pre>
Subtracting $1$ to get $M_{67}$:
{{begin-eqn}}
{{eqn | l = 2^{67} - 1
      | r = 147 \, 573 \, 952 \, 589 \, 676 \, 412 \, 928 - 1
      | c = 
}}
{{eqn | r = 147 \, 573 \, 952 \, 589 \, 676 \, 412 \, 927
      | c = 
}}
{{end-eqn}}
Now to calculate $193 \, 707 \, 721 \times 761 \, 838 \, 257 \, 287$:
<pre>
                193 707 721
          x 761 838 257 287
--------------------------
              1 355 954 047
             15 496 617 680
             38 741 544 200
          1 355 954 047 000
          9 685 386 050 000
         38 741 544 200 000
      1 549 661 768 000 000
      5 811 231 630 000 000
    154 966 176 800 000 000
    193 707 721 000 000 000
 11 622 463 260 000 000 000
135 595 404 700 000 000 000
---------------------------
147 573 952 589 676 412 927
---------------------------
  1 223 254 435 432 22  1
</pre>
Thus:
{{begin-eqn}}
{{eqn | l = 2^{67} - 1
      | r = 147 \, 573 \, 952 \, 589 \, 676 \, 412 \, 927
      | c = 
}}
{{eqn | r = 193 \, 707 \, 721 \times 761 \, 838 \, 257 \, 287
      | c = 
}}
{{end-eqn}}
as we were to demonstrate.
{{qed}}