Timeline for How often do two powers of 2 equal two powers of 10 (when summed)?
Current License: CC BY-SA 3.0
7 events
| when toggle format | what | by | license | comment | |
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| Apr 13, 2017 at 12:58 | history | edited | CommunityBot |
replaced http://mathoverflow.net/ with https://mathoverflow.net/
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| Apr 3, 2013 at 15:18 | vote | accept | Barry Cipra | ||
| Apr 3, 2013 at 1:16 | answer | added | Siksek | timeline score: 13 | |
| Apr 2, 2013 at 21:57 | answer | added | Mike Bennett | timeline score: 4 | |
| Apr 2, 2013 at 17:47 | comment | added | user30035 | [I'm not suggesting this helps, I'm just answering the implicit question in your last sentence.] | |
| Apr 2, 2013 at 17:46 | comment | added | user30035 | $a$ is determined by $n$ because $a$ is the $5$-adic valuation of $2^n+1$. More precisely, $2$ is a primitive root mod $5^t$ for all $t\geq1$ so $5^t$ divides $2^n+1$ iff $2^n=-1=2^{2.5^{t-1}}$ mod $5^t$ iff $n$ is congruent to $2.5^{t-1}$ mod $4.5^{t-1}$, so $a$ is the largest value of $t$ for which that's true. Basically $n$ has to be 2 mod 4 for $a>0$ and then $a$ is one plus the number of times 5 goes into $n$, if I got it right. | |
| Apr 2, 2013 at 16:06 | history | asked | Barry Cipra | CC BY-SA 3.0 |