diff --git a/base/src/functions/engineering/mod.rs b/base/src/functions/engineering/mod.rs
index 5a125ba..1f549cf 100644
--- a/base/src/functions/engineering/mod.rs
+++ b/base/src/functions/engineering/mod.rs
@@ -5,5 +5,3 @@ mod convert;
 mod misc;
 mod number_basis;
 mod transcendental;
-
-pub use transcendental::{fact, fact_double};
diff --git a/base/src/functions/engineering/transcendental/gamma.rs b/base/src/functions/engineering/transcendental/gamma.rs
deleted file mode 100644
index cffec59..0000000
--- a/base/src/functions/engineering/transcendental/gamma.rs
+++ /dev/null
@@ -1,106 +0,0 @@
-use std::f64::consts::PI;
-
-// FIXME: Please do a better approximation
-
-/// Lanczos approximation for Gamma(z).
-/// Valid for z > 0. For z <= 0 use `gamma` below (which does reflection).
-fn gamma_lanczos(z: f64) -> f64 {
-    const COEFFS: [f64; 9] = [
-        0.999_999_999_999_809_9,
-        676.5203681218851,
-        -1259.1392167224028,
-        771.323_428_777_653_1,
-        -176.615_029_162_140_6,
-        12.507343278686905,
-        -0.13857109526572012,
-        9.984_369_578_019_572e-6,
-        1.5056327351493116e-7,
-    ];
-
-    let g = 7.0;
-    let mut x = COEFFS[0];
-
-    // sum_{i=1}^{n} c_i / (z + i)
-    for (i, c) in COEFFS.iter().enumerate().skip(1) {
-        x += c / (z + i as f64);
-    }
-
-    let t = z + g + 0.5;
-    // √(2π) * t^(z+0.5) * e^{-t} * x
-    (2.0 * PI).sqrt() * t.powf(z + 0.5) * (-t).exp() * x
-}
-
-/// Gamma function for all real x (except poles),
-/// using Lanczos + reflection formula.
-fn gamma(x: f64) -> f64 {
-    if x.is_sign_negative() {
-        // Reflection formula: Γ(x) = π / (sin(πx) * Γ(1 - x))
-        // Beware of sin(πx) near zeros.
-        let sin_pix = (PI * x).sin();
-        if sin_pix == 0.0 {
-            return f64::NAN;
-        }
-        PI / (sin_pix * gamma_lanczos(1.0 - x))
-    } else if x == 0.0 {
-        f64::INFINITY
-    } else {
-        gamma_lanczos(x)
-    }
-}
-
-/// Factorial for real x: x! = Γ(x+1)
-pub fn fact(x: f64) -> f64 {
-    if x < 0.0 {
-        return f64::NAN;
-    }
-    gamma(x + 1.0)
-}
-
-/// Double factorial for real x.
-///
-/// Strategy:
-/// 1. If x is (almost) integer and >= 0 → do exact product (stable, no surprises)
-/// 2. Else:
-///    - If x is “even-ish”: x = 2t → x!! = 2^t * Γ(t+1)
-///    - If x is “odd-ish”:  x = 2t-1 → x!! = Γ(2t+1) / (2^t * Γ(t+1))
-pub fn fact_double(x: f64) -> f64 {
-    if x < 0.0 {
-        return f64::NAN;
-    }
-
-    let xi = x.round();
-    let is_int = (x - xi).abs() < 1e-9;
-
-    if is_int {
-        // integer path
-        let mut acc = 1.0;
-        let mut k = xi;
-        while k > 1.0 {
-            acc *= k;
-            k -= 2.0;
-        }
-        return acc;
-    }
-
-    // non-integer path: use continuous extension
-    let frac = x % 2.0;
-    // even-ish if close to 0 mod 2
-    if frac.abs() < 1e-6 || (frac - 2.0).abs() < 1e-6 {
-        // x = 2t
-        let t = x / 2.0;
-        2f64.powf(t) * gamma(t + 1.0)
-    } else if (frac - 1.0).abs() < 1e-6 || (frac + 1.0).abs() < 1e-6 {
-        // x = 2t - 1  => t = (x+1)/2
-        let t = (x + 1.0) / 2.0;
-        // (2t - 1)!! = Γ(2t + 1) / (2^t Γ(t+1))
-        // but 2t - 1 = x, so 2t + 1 = x + 2
-        let num = gamma(x + 2.0);
-        let den = 2f64.powf(t) * gamma(t + 1.0);
-        num / den
-    } else {
-        // x is neither clearly even-ish nor odd-ish – fall back to a generic formula.
-        // A safe thing to do is to treat it as even-ish:
-        let t = x / 2.0;
-        2f64.powf(t) * gamma(t + 1.0)
-    }
-}
diff --git a/base/src/functions/engineering/transcendental/mod.rs b/base/src/functions/engineering/transcendental/mod.rs
index d4d301f..9a90a48 100644
--- a/base/src/functions/engineering/transcendental/mod.rs
+++ b/base/src/functions/engineering/transcendental/mod.rs
@@ -5,7 +5,6 @@ mod bessel_jn_yn;
 mod bessel_k;
 mod bessel_util;
 mod erf;
-mod gamma;
 
 #[cfg(test)]
 mod test_bessel;
@@ -15,4 +14,3 @@ pub(crate) use bessel_jn_yn::jn as bessel_j;
 pub(crate) use bessel_jn_yn::yn as bessel_y;
 pub(crate) use bessel_k::bessel_k;
 pub(crate) use erf::erf;
-pub use gamma::{fact, fact_double};
diff --git a/base/src/functions/mathematical.rs b/base/src/functions/mathematical.rs
index e1e522e..549d9c6 100644
--- a/base/src/functions/mathematical.rs
+++ b/base/src/functions/mathematical.rs
@@ -2,7 +2,6 @@ use crate::cast::NumberOrArray;
 use crate::constants::{LAST_COLUMN, LAST_ROW};
 use crate::expressions::parser::ArrayNode;
 use crate::expressions::types::CellReferenceIndex;
-use crate::functions::engineering::{fact, fact_double};
 use crate::number_format::to_precision;
 use crate::single_number_fn;
 use crate::{
@@ -456,8 +455,35 @@ impl Model {
         Ok(1.0 / f64::cosh(f))
     });
     single_number_fn!(fn_exp, |f: f64| Ok(f64::exp(f)));
-    single_number_fn!(fn_fact, |f| Ok(fact(f)));
-    single_number_fn!(fn_factdouble, |f| Ok(fact_double(f)));
+    single_number_fn!(fn_fact, |x: f64| {
+        let x = x.floor();
+        if x < 0.0 {
+            return Err(Error::NUM);
+        }
+        let mut acc = 1.0;
+        let mut k = 2.0;
+        while k <= x {
+            acc *= k;
+            k += 1.0;
+        }
+        Ok(acc)
+    });
+    single_number_fn!(fn_factdouble, |x: f64| {
+        let x = x.floor();
+        if x < -1.0 {
+            return Err(Error::NUM);
+        }
+        if x < 0.0 {
+            return Ok(1.0);
+        }
+        let mut acc = 1.0;
+        let mut k = if x % 2.0 == 0.0 { 2.0 } else { 1.0 };
+        while k <= x {
+            acc *= k;
+            k += 2.0;
+        }
+        Ok(acc)
+    });
     single_number_fn!(fn_sign, |f| Ok(f64::signum(f)));
 
     pub(crate) fn fn_pi(&mut self, args: &[Node], cell: CellReferenceIndex) -> CalcResult {
diff --git a/base/src/functions/mod.rs b/base/src/functions/mod.rs
index 0841fa7..fcc5f9f 100644
--- a/base/src/functions/mod.rs
+++ b/base/src/functions/mod.rs
@@ -602,6 +602,11 @@ impl Function {
             "SECH" => Some(Function::Sech),
             "ACOTH" => Some(Function::Acoth),
 
+            "FACT" => Some(Function::Fact),
+            "FACTDOUBLE" => Some(Function::Factdouble),
+            "EXP" => Some(Function::Exp),
+            "SIGN" => Some(Function::Sign),
+
             "PI" => Some(Function::Pi),
             "ABS" => Some(Function::Abs),
             "SQRT" => Some(Function::Sqrt),
diff --git a/webapp/IronCalc/package-lock.json b/webapp/IronCalc/package-lock.json
index 8df1630..f782380 100644
--- a/webapp/IronCalc/package-lock.json
+++ b/webapp/IronCalc/package-lock.json
@@ -40,7 +40,7 @@
     },
     "../../bindings/wasm/pkg": {
       "name": "@ironcalc/wasm",
-      "version": "0.5.0",
+      "version": "0.6.0",
       "license": "MIT/Apache-2.0"
     },
     "node_modules/@adobe/css-tools": {
diff --git a/xlsx/tests/calc_tests/FACT_DOUBLEFACT.xlsx b/xlsx/tests/calc_tests/FACT_DOUBLEFACT.xlsx
new file mode 100644
index 0000000..04e814f
Binary files /dev/null and b/xlsx/tests/calc_tests/FACT_DOUBLEFACT.xlsx differ