There's something special about Ramanujan! -- part 2

Logic Level 3

The first term of a sequence is 1111. Each succeeding term is the sum of the cubes of the digits of the previous term. What is the 1729-th term of the sequence?


Inspiration .


The answer is 55.

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4 solutions

We can see that,

a 1 = 1111 a_{1} = 1111

a 2 = 1 3 + 1 3 + 1 3 + 1 3 = 4 a_{2} = 1^3+1^3+1^3+1^3 = 4

a 3 = 4 3 = 64 a_{3} = 4^3=64

a 4 = 6 3 + 4 3 = 280 a_{4} = 6^3+4^3 =280

a 5 = 2 3 + 8 3 + 0 3 = 520 a_{5} = 2^3+8^3+0^3 = 520

a 6 = 5 3 + 2 3 + 0 3 = 133 a_{6} = 5^3+2^3+0^3 = 133

a 7 = 1 3 + 3 3 + 3 3 = 55 a_{7} = 1^3 + 3^3+3^3 = 55

a 8 = 5 3 + 5 3 = 250 a_{8} = 5^3 + 5^3 = 250

a 9 = 2 3 + 5 3 + 0 3 = 133 a_{9} = 2^3+5^3 + 0^3 = 133

and the series continues so on. The 172 8 t h o r a 1728 1728^{th} \, or \, a_{1728} term will be the same as a 6 a_{6} so the 172 9 t h o r a 1729 1729^{th} \, or \,a_{1729} term will be the same as a 7 a_{7} a 1729 = a 7 = 55 a_{1729} = a_{7} = \boxed {55}

1
2
1723 mod 3 = 1
1729 is a taxicab number

Aditya Chauhan
Aug 27, 2015

From the 6th term the series is repeating the order 133 , 55 , 250 133,55,250\dots\dots

So the 1729 t h 1729th term will be equal to 55 \boxed{55}

Vraj Mistry
Aug 26, 2015

the series is like 1111,4,64,280,520,133,55,250,133,55,250....................................and so on

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