Solution 1
Let f(x)=log(x+f(x−1)) and f(2)=log(2), and from the problem description, A=f(2013)
We can reason out an approximation, by ignoring the f(x−1):
f0(x)≈logx
And a better approximation, by plugging in our first approximation for f(x−1) in our original definition for f(x):
f1(x)≈log(x+log(x−1))
And an even better approximation:
f2(x)≈log(x+log(x−1+log(x−2)))
Continuing this pattern, obviously, will eventually terminate at fx−1(x), in other words our original definition of f(x).
However, at x=2013, going further than f1(x) will not distinguish between our answer choices. log(2012+log(2011)) is nearly indistinguishable from log(2012).
So we take f1(x) and plug in.
f(2013)≈log(2013+log2012)
Since 10002017, then log(2012+log(2011+⋯))>4.
So 2012+log(2011+⋯)>10000.
Repeating, we then get 2011+log(2010+⋯)>107988.
This is clearly absurd (the RHS continues to grow more than exponentially for each iteration).
So, x is not greater than 2017.
So A2. We are looking for f(2013). First we show
Lemma. For any integer n>2, if n2. We start with a simple observation:
Lemma. For x,y>2, log(x+log(y))2, we have xy−x−y=(x−1)(y−1)−1>0, so xy−x−log(y)>xy−x−y>0.
It follows that log(z+log(x+log(y)))<log(x)+log(y)+log(z), and so on.
Thus f(2010)<log2+log3+⋯+log2010<log2010+⋯+log2010<2009⋅4=8036.
Then f(2011)=log(2011+f(2010))<log(10047)<5.
It follows that f(2012)=log(2012+f(2011))<log(2017)<4.
Finally, we get f(2013)=log(2013+f(2012))<log(2017), which leaves us with only option (A).
Solution 5 (nonrigorous + abusing answer choices.)
Intuitively, you can notice that log(2012+log(2011+⋯(log(2)⋯))<log(2013+log(2012+⋯(log(2))⋯)), therefore (by the answer choices) log(2012+log(2011+⋯(log(2)⋯))<log(2021). We can then say:
x=log(2013+log(2012+⋯(log(2))⋯))
log(2013)<x<log(2013+log(2021))
log(2013)<x<log(2013+4)
log(2013)<x<log(2017)
The only answer choice that is possible given this information is (A)
Solution 6 (super quick)
Let f(x)=log(x+log((x−1)+log((x−2)+…+log2…))). From the answer choices, we see that 3<f(2013)<4. Since f(x) grows very slowly, we can assume 3<f(2012)<4. Therefore, f(2013)=log(2013+f(2012))∈(log2016,log2017)⟹(A).
~rayfish
Solution 7 (Not rigorous but it works)
Start small from log(3+log2). Because log2 is close to 0, the expression is close to log3. This continues for the 1 digit numbers. However, when we get to log(11+log10), log10=1 so the expression equals log12. We can round down how much each new log contributes to the expression as an approximate answer. There are 90 2 digit numbers, 900 3 digit numbers, and 1014 4 digit numbers that increase the expression. 1000<90⋅1+900⋅2+1014⋅3<10000, so it increases between 3 and 4. The answer is (A).
~dragnin
Video Solution by Richard Rusczyk
https://artofproblemsolving.com/videos/amc/2013amc12a/360