Fix/sum preview

app/preview_test.py

@@ -273,6 +273,47 @@ def test_laplace_transforms(self, response, is_latex, latex, sympy):
         assert result["preview"]["latex"] == latex
         assert result["preview"]["sympy"] == sympy
 
+    @pytest.mark.parametrize(
+        "response, latex, sympy", [
+            # summation() with explicit bounds — rendered correctly by SymPy
+            (
+                'summation((4/n**2)(-1)**n cos(nx), (n, 1, oo))',
+                r'\sum_{n=1}^{\infty} \frac{4 \cdot \left(-1\right)^{n} \cdot \cos{\left(n \cdot x \right)}}{n^{2}}',
+                'summation((4/n**2)(-1)**n cos(nx), (n, 1, oo))',
+            ),
+            # infsum as a custom prefix symbol — content must appear inside the sum, not outside
+            (
+                'infsum((-1)^n / n^2)',
+                r'\sum_{n=1}^{\infty} \frac{\left(-1\right)^{n}}{n^{2}}',
+                'infsum((-1)^n / n^2)',
+            ),
+            (
+                'infsum((4/n^2)*(-1)^n*cos(nx))',
+                r'\sum_{n=1}^{\infty} \frac{4 \cdot \left(-1\right)^{n} \cdot \cos{\left(n \cdot x \right)}}{n^{2}}',
+                'infsum((4/n^2)*(-1)^n* cos(nx))',
+            ),
+            (
+                '((pi^2)/(3))+infsum((4/n^2)*(-1)^n*cos(nx))',
+                r'\frac{\pi^{2}}{3} + \sum_{n=1}^{\infty} \frac{4 \cdot \left(-1\right)^{n} \cdot \cos{\left(n \cdot x \right)}}{n^{2}}',
+                '(( pi^2)/(3))+infsum((4/n^2)*(-1)^n* cos(nx))',
+            ),
+        ]
+    )
+    def test_sum_preview(self, response, latex, sympy):
+        params = {
+            "is_latex": False,
+            "strict_syntax": False,
+            "elementary_functions": True,
+            "symbols": {
+                "infsum": {"aliases": [], "latex": "\\sum_{n=1}^{\\infty}"},
+                "pi": {"aliases": [], "latex": "\\pi"},
+            },
+        }
+
+        result = preview_function(response, params)
+        assert result["preview"]["latex"] == latex
+        assert result["preview"]["sympy"] == sympy
+
 
 
 if __name__ == "__main__":

app/utility/expression_utilities.py

@@ -561,22 +561,33 @@ class SymbolData(TypedDict):
     r"(?P<start>\\\(|\$\$|\$)(?P<latex>.*?)(?P<end>\\\)|\$\$|\$)"
 )
 
+BIG_OPERATOR_PREFIXES = (r"\sum", r"\int", r"\prod", r"\lim", r"\bigcup", r"\bigcap")
+
 
 def sympy_symbols(symbols):
     """Create a mapping of local variables for parsing sympy expressions.
 
     Args:
         symbols (SymbolDict): A dictionary of sympy symbol strings to LaTeX
-        symbol strings.
-
-    Note:
-        Only the sympy string is used in this function.
+        symbol strings, or an iterable of symbol name strings.
 
     Returns:
-        Dict[str, Symbol]: A dictionary of sympy symbol strings to sympy
-        Symbol objects.
+        Dict[str, Symbol | type]: A dictionary mapping symbol names to SymPy
+        Symbol objects, or to Function subclasses for prefix operator symbols
+        (those whose latex begins with a big operator command like \\sum).
     """
-    return {k: Symbol(k) for k in symbols}
+    result = {}
+    for k in symbols:
+        symbol_def = symbols[k] if isinstance(symbols, dict) else None
+        if isinstance(symbol_def, dict):
+            latex = extract_latex(symbol_def.get("latex", "")).strip()
+            if any(latex.startswith(op) for op in BIG_OPERATOR_PREFIXES):
+                def _latex(self, printer, _l=latex):
+                    return _l + " " + printer.doprint(self.args[0])
+                result[k] = type(k, (Function,), {"_latex": _latex})
+                continue
+        result[k] = Symbol(k)
+    return result
 
 
 def extract_latex(symbol):