xpath question

i want to access to the preceding node N1 from a node N2
and also its  following node N3
so i use this instruction:
for N1 :  $N2/preceding-sibling::node()
for N3 :  $N2/following-sibling::node()
but it's not correct,i don't know why,

my xml file is like:

.......
<N1>
<N2>
<N3>
.......



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