diff --git a/base/special/gamma-lanczos-sum/README.md b/base/special/gamma-lanczos-sum/README.md
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--- a/base/special/gamma-lanczos-sum/README.md
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+* * *
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+## C APIs
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+### Usage
+
+```c
+#include "stdlib/math/base/special/gamma_lanczos_sum.h"
+```
+
+#### gamma_lanczos_sum( x )
+
+Calculates the Lanczos sum for the approximation of the [gamma function][gamma-function].
+
+```c
+double out = gamma_lanczos_sum( 4.0 );
+// returns ~950.366
+
+out = gamma_lanczos_sum( -1.5 );
+// returns ~1373366.245
+```
+
+The function accepts the following arguments:
+
+- **x**: `[in] double` input value.
+
+```c
+double gamma_lanczos_sum( const double x );
+```
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+### Examples
+
+```c
+#include "stdlib/math/base/special/gamma_lanczos_sum.h"
+#include
+#include
+
+int main( void ) {
+ const double x[] = { 4.0, -1.5, -0.5, 0.5 };
+
+ double y;
+ int i;
+ for ( i = 0; i < 4; i++ ) {
+ y = stdlib_base_gamm_lanczos_sum( x[ i ] );
+ printf( "gamm_lanczos_sum(%lf) = %lf\n", x[ i ], y );
+ }
+}
+```
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