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Formula Formulation (Posted on 2014-03-03) Difficulty: 3 of 5
A function f(N) is defined as:
f(1)=sod(12), f(2) = sod(122)
f(3) = sod(1212), f(4) = sod(12122), .... and, so on

Derive the general formula for f(N) in terms of N.

*** sod(x) denotes the sum of the digits of base ten expansion of x.

No Solution Yet Submitted by K Sengupta    
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Question Clarification | Comment 1 of 4

When you refer to the sum of the digits, is it only one iteration, or is it repeated until the sum is a single digit (I believe this is called the digital root)?

For example, 121 = 14,641.  Is sod(121) = 16, or is it 7?


  Posted by tomarken on 2014-03-03 16:25:03
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