Triangles within triangles

Consider the following number S = n = 1 37 9 ( 3 7 n ) S=\sum _{ n=1 }^{ 37 }{ 9(37^{ n }) } .

Suppose K K is the number of non-zero elements of row S + 1 S+1 of Pascal's triangle in modulo 37 37 . (To make this clear, row S + 1 S+1 would be the row with S + 1 S+1 elements) What is log 10 K \log _{ 10 }{ K } ?


The answer is 37.

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1 solution

Kartik Sharma
Feb 26, 2015

Well, this question looks difficult(at least to me) at first sight but it is very easy(including that easy guess).

Well, we will not first solve the sum.

In Pascal's triangle, rows consist of numbers of the form C ( n 1 , k ) C(n-1,k) for row n n and for k = [ 0 , n 1 ] k = [0,n-1] .

In ( S + 1 ) (S+1) th row, the n 1 n-1 would be just equal to S S .

Next, we have to find C ( S , k ) m o d 37 C(S,k) mod 37 for k = [ 0 , S ] k = [0,S] (hence S + 1 S+1 elements)

Now, Lucas Theorem says C ( n , r ) m o d p C(n,r) mod p is the product of combination operators of the corresponding coefficients of the base p p expansion of n n and r r .

S = 9 37 37 + 9 37 36 + . . . . . + 9 37 + 0 37 S = 9*{37}^{37} + 9*{37}^{36} +..... + 9*37 + 0*37

and k = n 37 p 37 + n 36 p 36 + . . . . + n 1 p + n 0 k = {n}_{37}{p}^{37} + {n}_{36}{p}^{36} + .... + {n}_{1}p + {n}_{0}

Hence, C ( S , k ) = C ( 9 , n 37 ) C ( 9 , n 36 ) C ( 9 , n 35 ) . . . . C ( 9 , n 1 ) C ( 0 , n 0 ) m o d 37 C(S,k) = C(9,{n}_{37})*C(9,{n}_{36})*C(9,{n}_{35})....C(9,{n}_{1})*C(0,{n}_{0}) mod 37

Now the remainder side will be non-zero if n k 9 {n}_{k} \leq 9 (except for n 0 {n}_{0} when it has to be equal to 0 0 only because if it is greater than 0 0 , the operator becomes 0 0 and i f it is less than 0 0 , it again becomes 0 0 . So,

n k = [ 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ] {n}_{k} = [0,1,2,3,4,5,6,7,8,9] (for k = [ 1 , 37 ] k = [1, 37] )

and n 0 = 0 {n}_{0} = 0

As a result, we can use Law of Multiplication of counting and get the final answer as - 10 37 {10}^{37} which will be followed by l o g 10 37 = 37 log {10}^{37} = \boxed{37}

Nice problem!

Kartik Sharma - 6 years, 3 months ago

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When I made this problem I was actually unaware of Lucas Theorem, but then Calvin Lin brought it up. I'll make a quick outline of my solution:

First, you show that row p + 1 p+1 is all 0's (except for the two 1's on the side, of course) iff p p is prime. Then, use induction to get the "triangles within triangles" the problem hinted at. (Image: http://www-math.ucdenver.edu/~wcherowi/pt7.gif)

You can show that to get a larger triangle you need p p rows of smaller triangles, because you can "add" the smaller triangles like they are elements of Pascal's triangle. (Thus, you get p p rows of such triangles from the earlier result).

Once you have this pattern, you consider the base p p expansion of n n and you realize that for every '1' you multiply by 2 (because then row n n contains two smaller triangles; then you consider an individual smaller triangle.), and so on until p 1 p-1 . You get the same result as Lucas Theorem gives you.

Dylan Pentland - 6 years, 3 months ago

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