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Is there some function in Mathematica which returns {c, i, j} for an input c[i,j]?

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A simple approach is

c[i, j];
{Head[%], %1[[1]], %[[2]]}
(* {c, i, j} *)

Slightly more generally, use

f[x_] := Join[{Head[x]}, List @@ x]
f[c[i, j]]
(* {c, i, j} *)
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    $\begingroup$ {Head[#], Sequence @@ #} &[c[i, j]] also works. $\endgroup$ – J. M. is away Aug 19 '16 at 19:39
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Another possible way using Level and Heads.

Level[c[i, j], 1, Heads -> True]
(* {c, i, j} *)
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using pattern-matching/deconstruction

c[i, j] /. a_[b_, c_] -> {a, b, c}

or define your own function:

f[a_[b_, c_]]:={a, b, c}
f[  c[i, j] ]  

{c, i, j}

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