how to find the asymptotic growth of f











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The question is:
Let $$S_n=left{tinmathbb{N}mid ttext{ does not contain }ntext{ consectuive }4'text{s}right}$$ E.g. $2464in S_2$, but $2544$ is not.



Let $$f(n)=sum_{sin S_n}frac{1}{s}$$




Prove that $f(n)$ is finite and find the asymptotic growth of $f$.




I have tried to find out the number of $d$-digit numbers in $S_2$ for any integer $d$. Then approximate the value of the inverse of all these $d$-digit numbers in $S_2$. Then generalize the number of $d$-digit numbers in $S_n$ for any integer $d,n$.



I find it hard to find the number of $d$-digit numbers in $S_3$ for any integer $d$. Because $S_3$ allows the numbers to have two places having $2$ consecutive $4$'s, like $44544,544344$ and $4454$ is also in $S_3$.



Then I run out of ideas. How can I do this question?










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    Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
    – José Carlos Santos
    Nov 25 at 14:15















up vote
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down vote

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The question is:
Let $$S_n=left{tinmathbb{N}mid ttext{ does not contain }ntext{ consectuive }4'text{s}right}$$ E.g. $2464in S_2$, but $2544$ is not.



Let $$f(n)=sum_{sin S_n}frac{1}{s}$$




Prove that $f(n)$ is finite and find the asymptotic growth of $f$.




I have tried to find out the number of $d$-digit numbers in $S_2$ for any integer $d$. Then approximate the value of the inverse of all these $d$-digit numbers in $S_2$. Then generalize the number of $d$-digit numbers in $S_n$ for any integer $d,n$.



I find it hard to find the number of $d$-digit numbers in $S_3$ for any integer $d$. Because $S_3$ allows the numbers to have two places having $2$ consecutive $4$'s, like $44544,544344$ and $4454$ is also in $S_3$.



Then I run out of ideas. How can I do this question?










share|cite|improve this question




















  • 1




    Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
    – José Carlos Santos
    Nov 25 at 14:15













up vote
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down vote

favorite
1









up vote
-1
down vote

favorite
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1





The question is:
Let $$S_n=left{tinmathbb{N}mid ttext{ does not contain }ntext{ consectuive }4'text{s}right}$$ E.g. $2464in S_2$, but $2544$ is not.



Let $$f(n)=sum_{sin S_n}frac{1}{s}$$




Prove that $f(n)$ is finite and find the asymptotic growth of $f$.




I have tried to find out the number of $d$-digit numbers in $S_2$ for any integer $d$. Then approximate the value of the inverse of all these $d$-digit numbers in $S_2$. Then generalize the number of $d$-digit numbers in $S_n$ for any integer $d,n$.



I find it hard to find the number of $d$-digit numbers in $S_3$ for any integer $d$. Because $S_3$ allows the numbers to have two places having $2$ consecutive $4$'s, like $44544,544344$ and $4454$ is also in $S_3$.



Then I run out of ideas. How can I do this question?










share|cite|improve this question















The question is:
Let $$S_n=left{tinmathbb{N}mid ttext{ does not contain }ntext{ consectuive }4'text{s}right}$$ E.g. $2464in S_2$, but $2544$ is not.



Let $$f(n)=sum_{sin S_n}frac{1}{s}$$




Prove that $f(n)$ is finite and find the asymptotic growth of $f$.




I have tried to find out the number of $d$-digit numbers in $S_2$ for any integer $d$. Then approximate the value of the inverse of all these $d$-digit numbers in $S_2$. Then generalize the number of $d$-digit numbers in $S_n$ for any integer $d,n$.



I find it hard to find the number of $d$-digit numbers in $S_3$ for any integer $d$. Because $S_3$ allows the numbers to have two places having $2$ consecutive $4$'s, like $44544,544344$ and $4454$ is also in $S_3$.



Then I run out of ideas. How can I do this question?







summation permutations asymptotics infinity






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share|cite|improve this question













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edited Nov 25 at 14:26

























asked Nov 25 at 14:02









idunno

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71








  • 1




    Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
    – José Carlos Santos
    Nov 25 at 14:15














  • 1




    Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
    – José Carlos Santos
    Nov 25 at 14:15








1




1




Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
– José Carlos Santos
Nov 25 at 14:15




Welcome to MSE. For some basic information about writing mathematics at this site see, e.g., basic help on mathjax notation, mathjax tutorial and quick reference, main meta site math tutorial and equation editing how-to.
– José Carlos Santos
Nov 25 at 14:15















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