What's the best way to write subsequences? `$x_n_i$`

gives an error, while `$x_{n_{i}}$`

works, but it just looks like `$x_{ni}$`

(the `i`

isn't lowered enough below the `n`

)…

# [Tex/LaTex] Double Subscript for Subsequences

subscripts

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# [Tex/LaTex] Double Subscript for Subsequences

###### Related Question

subscripts

`$x_n_i$`

gives an error, while `$x_{n_{i}}$`

works, but it just looks like `$x_{ni}$`

(the `i`

isn't lowered enough below the `n`

)…

## Best Answer

I'm a bit puzzled by your statement that

`$x_{n_i}$`

would create output which "just looks like" that of`$x_{ni}$`

-- this is not the case in the following MWE (minimum working example). I have a hunch that what you want is`$x_{n_i}$`

, but read on.Clearly, the first expression,

`$x_{ni}$`

, can't be what you want. Note that`${x_n}_i$`

and`$x_{n_i}$`

are both valid expressions from a purely syntactic point of view. However, they donotcreate the same output. In the former case, the character`i`

is bothlowered by a smaller amountand has alarger font sizethan is the case in the latter. (To be a bit TeXnical, in the first two expressions above,`n`

and`i`

are both typeset in "scriptstyle", whereas in the third expression`n`

is in scriptstyle and`i`

is in "scriptscriptstyle". For Computer Modern math fonts, "scriptstyle" is 30% linearly reduced from "textstyle", and "scriptscriptstyle" is 30% reduced from "scriptstyle" -- or ca 50% linearly reduced from "textstyle". Thus, if the textstyle font size is 10pt, scriptsize is 7pt and scriptscriptsize is 5pt.)These differences in appearance are, of course, not accidental: in the middle expression above, the symbol

`i`

is a subscript/index to the subformula`$x_n$`

, whereas in the final expression`i`

indexes`n`

which, in turn, indexes`x`

. Put differently, in the final expression`${n_i}$`

is a subformula that serves to index`x`

.If there's

anychance for ambiguity as to which characters are supposed to index which other characters, you should not hesitate to use parentheses, brackets, or braces -- or whatever grouping symbols are appropriate in your math writing style -- to clarify the intended meaning of your writing.Addendum, prompted by a follow-up communication from the OP. As the image above illustrates, there's not much visual difference in the appearance of the first and second cases, i.e., of`$x_{ni}$`

and`${x_n}_i$`

. In both cases, (a) the`n`

and`i`

characters are in`scriptsize`

, and (b) the`i`

's are set below the baseline by the same amount. The only visual difference, then, is that TeX sets the`n`

in the second case in so-called "cramped subscript" mode, which differs from the "ordinary subscript" position by a (very) small amount.