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 Two Power Divides Product (Posted on 2011-12-05)
Find n where 2n is the largest power of 2 that divides the product 2011*2012*2013*.....*4022

 No Solution Yet Submitted by K Sengupta Rating: 4.0000 (2 votes)

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 re(2): analytic solution and computer solution | Comment 5 of 8 |
(In reply to re: analytic solution and computer solution by Ady TZIDON)

2011 is 27 mod 64 and 4022 is 54 mod 64, so that of the 2012 numbers in the given range, [2012/64] = 31 are divisible by 64, bringing the total to 1980.

Here are 31; what other is there?

2048  2112  2176  2240  2304  2368  2432  2496  2560  2624  2688  2752  2816
2880  2944  3008  3072  3136  3200  3264  3328  3392  3456  3520  3584  3648
3712  3776  3840  3904  3968

Likewise, here are 16 divisible by 128; where is the 17th?

2048  2176  2304  2432  2560  2688  2816  2944  3072  3200  3328  3456  3584
3712  3840  3968

The eight that are divisible by 256 are here; where it the 9th?

2048  2304  2560  2816  3072  3328  3584  3840

Finally:

2011 is 475 mod 512 and 4022 is 438 mod 512, so that of the 2012 numbers in the given range, ceil(2012/512) = 4 are divisible by 512, bringing the total to 2008.

SHOULD BE 5 INSTEAD OF 4

I LIST THEM:  1544, 2056, 2568, 3080, 3592

but 1544 is not in the range and none of these is divisible by 512. The actual ones:

2048  2560  3072  3584

 Posted by Charlie on 2011-12-06 01:47:49

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