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Find the number of injective functions (from finite set to finite set):

$ f:\{ 1,2,3,4,5,6,7\} \rightarrow \{ 1,2,3,4,5,6,7,8,9\} $

with the following property: $ f(i) \neq f(j)+1\ \text{ for all } \ 1\leq i < j \leq 7. $

Help me with mathematical decision (I can count this number on computer). Thank you.

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    If you find the answer, you are encouraged to write it up as an answer and accept it.2012-11-15

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