The Powers of 32 32 !

One day for no apparent reason, Brilli the ant decides to add up different non-negative powers of 32 32 and begins to write them down in ascending order. The first few terms of her sequence look like this:

1 , 32 , 33 , 1024 , 1025 , 1056 , 1057 , 32768 , 32769 , 32800 , 32801... 1, 32, 33, 1024, 1025, 1056, 1057, 32768, 32769, 32800, 32801...

If 35467067393 35467067393 is the N th N^{\text{th}} term of this sequence, what is N N ?

Details and assumptions:

A term can be the sum of more than two different powers of 32 32 . For example: 32801 = 3 2 3 + 3 2 1 + 3 2 0 32801=32^3+32^1+32^0 .

A term can be the "sum" of one number. For example: 1024 = 3 2 2 1024=32^2 is a term of this sequence.

A term, however, can't be the "sum" of zero numbers. In other words, 0 0 isn't in this sequence.

A term also has to be the sum of distinct numbers. That means 65 = 3 2 1 + 3 2 1 + 3 2 0 65=32^1+32^1+32^0 is not in the sequence.

This problem was inspired by a problem posed in the BdMO-2013 divisional round.


The answer is 233.

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3 solutions

Thanic Samin
Jan 22, 2014

If we rewrite the numbers in the base of 32, we get the consecutive binary natural numbers.

( 35467067393 ) 10 = ( 11101001 ) 32 (35467067393)_{10}=(11101001)_{32} ,So the answer is ( 11101001 ) 2 = ( 233 ) 10 (11101001)_2=(233)_{10}

Jubayer Nirjhor
Jan 21, 2014

The distinctness of the powers and a specific base instantly forces one to think about base conversions. We try base 32 32 and rewrite the sequence:

1 , 10 , 11 , 100 , 101 , . . . 1,10,11,100,101,...

It's the list of consecutive binary integers (it's obvious since the expansion of terms don't have any additional coefficients). The rest is straightforward.

35467067393 = 1110100 1 32 35467067393=11101001_{32}

1110100 1 2 = 233 11101001_2=233

N = 233 \therefore ~~~ N=\fbox{233}

Ben Frankel
Jan 21, 2014

Let's write this sequence in a different way.. we will convert each number to base 32:

[ 1 , 10 , 11 , 100 , 101 , 110 , 111 , 1000 , 1001 , . . . ] [1, 10, 11, 100, 101, 110, 111, 1000, 1001, ...]

Those familiar with binary should recognize a very clear pattern. These are the consecutive natural numbers in base 2!

After some calculations (find the greatest n n such that 3 2 n < 35467067393 32^n < 35467067393 , subtract and repeat), it can be found that 35467067393 11101001 35467067393 \equiv 11101001 when represented in base 32. To calculate this number's place in the sequence, we simply convert 11101001 11101001 to decimal, as if it were binary.

1 + 8 + 32 + 64 + 128 = 233 1 + 8 + 32 + 64 + 128 = \boxed{233}

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