Solution:
Note that
210n=1024n=1.024n×103n
So 210n has roughly 3n+1 digits for relatively small n's. (Actually we have that for 0<x<1,
(1+x)2=1+2x+x2<1+3x
Therefore, 1.0242<1.032<1.09, 1.092<1.27, 1.272<1.81<2, and 22=4, so 1.02416<4. Thus the conclusion holds for n≤16.)
For any positive integer n≤16,
A=i=1∑n210i×10∑j=i+1n(3j+1)
Let
Ai=210i×10∑j=i+1n(3j+1)
for 1≤i≤n, then we know that
A−210n=i=1∑n−1Ai
and
Ai=210i+∑j=i+1n(3j+1)×5∑j=i+1n(3j+1)=2ui×5vi
where ui=10i+∑j=i+1n(3j+1), vi=∑j=i+1n(3j+1). We have that
ui−ui−1=10−(3i+1)=3(3−i)
Thus, for 1≤i≤n−1, ui is minimized when i=1 or i=n−1, with u1=23n2+5n+12 and un−1=13n−9. When n=5,
A−210n=A1+A2+A3+A4=210×1046+220×1039+230×1029+240×1016
is at most divisible by 257 instead of 2170. For all other n's, we have that u1=un−1, so we should have that both 170≤u1 and 170≤un−1. Therefore, since 170≤un−1, we have that 14≤n. We can see that u1>170 and 14<16 in this case. Therefore, the minimum of n is 14.