First, we point out two obvious facts:
(a) If m is a positive integer and x is real, then
⌊mx⌋=⌊m⌊x⌋⌋.
(b) For any integer l and positive even number m, we have
⌊m2l+1⌋=⌊m2l⌋.
Let m=k! (k=1,2,…,2013) in (a) and summing up, we have
f(x)=k=1∑2013⌊k!x⌋=k=1∑2013⌊k!⌊x⌋⌋=f(⌊x⌋),
that is, f(x)=n has a real solution if and only if f(x)=n has an integer solution. So, we only consider x as an integer. Since
f(x+1)−f(x)=⌊x+1⌋−⌊x⌋+k=2∑2013(⌊k!x+1⌋−⌊k!x⌋)≥1,1◯
we see that f(x) (x∈Z) is monotonously increasing. Now, we find integers a and b, such that
f(a−1)<0≤f(a)<f(a+1)<…<f(b−1)<f(b)≤2013<f(b+1).
Note that f(−1)<0=f(0), so a=0. Since
f(1173)=k=1∑6⌊k!1173⌋=1173+586+195+48+9+1=2012≤2013,
f(1174)=k=1∑6⌊k!1174⌋=1174+587+195+48+9+1=2014>2013,
we see that b=1173.
So the good numbers in {1,3,5,...,2013} are the odd numbers in
{f(0),f(1),…,f(1173)}.
Let x=2l (l=0,1,…,586) in ①. By (b), we have
⌊k!2l+1⌋=⌊k!2l⌋(2≤k≤2013).
Thus,
f(2l+1)−f(2l)=1+k=2∑2013(⌊k!2l+1⌋−⌊k!2l⌋)=1,
that is, there is exactly one odd number in f(2l) and f(2l+1).
Therefore, there are 21174=587 odd numbers in {f(0),f(1),…,f(1173)}, that is, there are 587 good numbers in the set {1,3,5,…,2013}.
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