Maths Olympiad Prep

Track / Stage 6 / 259 of 400 #1259 of 1964

Problem 1259

National olympiad, first round
Number theory Difficulty 6.4 Find the answer

Find the number of integers kk in the set {0,1,2,,2012}\{0, 1, 2, \dots, 2012\} such that (2012k)\binom{2012}{k} is a multiple of 20122012.

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Official solution

To find the number of integers k k in the set {0,1,2,,2012}\{0, 1, 2, \dots, 2012\} such that (2012k)\binom{2012}{k} is a multiple of 20122012, we need to consider the prime factorization of 20122012.

First, we factorize 20122012:
2012=22×503 2012 = 2^2 \times 503

Thus, (2012k)\binom{2012}{k} is a multiple of 20122012 if and only if it is a multiple of both 44 and 503503.

### Step 1: Condition for (2012k)\binom{2012}{k} to be a multiple of 44

We use the pp-adic valuation, specifically ν2\nu_2, which counts the number of factors of 22 in a number. For (2012k)\binom{2012}{k} to be a multiple of 44, we need:
ν2((2012k))2 \nu_2\left(\binom{2012}{k}\right) \geq 2

Using the property of pp-adic valuation for binomial coefficients:
ν2((2012k))=ν2(2012!)ν2(k!)ν2((2012k)!) \nu_2\left(\binom{2012}{k}\right) = \nu_2(2012!) - \nu_2(k!) - \nu_2((2012-k)!)

From the lemma, ν2(n!)=nB(n)\nu_2(n!) = n - B(n), where B(n)B(n) is the number of 1's in the binary representation of nn. Therefore:
ν2(2012!)=2012B(2012) \nu_2(2012!) = 2012 - B(2012)
ν2(k!)=kB(k) \nu_2(k!) = k - B(k)
ν2((2012k)!)=(2012k)B(2012k) \nu_2((2012-k)!) = (2012 - k) - B(2012 - k)

Thus:
ν2((2012k))=(2012B(2012))(kB(k))((2012k)B(2012k)) \nu_2\left(\binom{2012}{k}\right) = (2012 - B(2012)) - (k - B(k)) - ((2012 - k) - B(2012 - k))
=2012B(2012)k+B(k)2012+k+B(2012k) = 2012 - B(2012) - k + B(k) - 2012 + k + B(2012 - k)
=B(k)+B(2012k)B(2012) = B(k) + B(2012 - k) - B(2012)

For (2012k)\binom{2012}{k} to be a multiple of 44:
B(k)+B(2012k)B(2012)2 B(k) + B(2012 - k) - B(2012) \geq 2

### Step 2: Binary Representation Analysis

The binary representation of 20122012 is:
2012=111110111002 2012 = 11111011100_2
Thus, B(2012)=8B(2012) = 8.

We need:
B(k)+B(2012k)10 B(k) + B(2012 - k) \geq 10

### Step 3: Counting kk such that B(k)+B(2012k)10B(k) + B(2012 - k) \geq 10

We need to count the number of kk such that B(k)+B(2012k)10B(k) + B(2012 - k) \geq 10. This involves considering the cases where there is carrying in the binary addition of kk and 2012k2012 - k.

### Step 4: Condition for (2012k)\binom{2012}{k} to be a multiple of 503503

For (2012k)\binom{2012}{k} to be a multiple of 503503, kk must be a multiple of 503503. The possible values of kk are:
k=0,503,1006,1509,2012 k = 0, 503, 1006, 1509, 2012

### Step 5: Combining Conditions

We need to combine the conditions from steps 1 and 4. We have:
- 512512 values of kk from the binary condition.
- 33 values of kk from the multiple of 503503 condition.

Thus, the total number of kk that fail both conditions is:
512+3=515 512 + 3 = 515

### Conclusion

The number of integers kk such that (2012k)\binom{2012}{k} is a multiple of 20122012 is:
2013515=1498 2013 - 515 = 1498

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.