diff --git a/build/math.js b/build/math.js
index f51f281..7c5f9d2 100644
--- a/build/math.js
+++ b/build/math.js
@@ -5237,13 +5237,13 @@ function discreteIntegral( values, step ) {
 }
 
 
-function summation( f, [a,b] ) {
+function summation( f, [a,b,step=1] ) {
 
   if ( isComplex( f(a) ) ) {
 
     var s = complex(0);
 
-    for ( var i = a ; i <= b ; i++ ) s = add( s, f(i) );
+    for ( var i = a ; i <= b ; i += step ) s = add( s, f(i) );
 
     return s;
 
@@ -5251,7 +5251,7 @@ function summation( f, [a,b] ) {
 
     var s = 0;
 
-    for ( var i = a ; i <= b ; i++ ) s += f(i);
+    for ( var i = a ; i <= b ; i += step ) s += f(i);
 
     return s;
 
diff --git a/docs/calculus.html b/docs/calculus.html
index a506d9b..a818aa5 100644
--- a/docs/calculus.html
+++ b/docs/calculus.html
@@ -45,7 +45,9 @@ <h1>Integration</h1>
 
 <p><b>discreteIntegral( values, step )</b> &mdash; numerical integral over discrete real values separated by step using Euler-Maclaurin summation</p>
 
-<p><b>summation( <i>f</i>, [<i>a</i>,<i>b</i>] )</b> &mdash; discrete summation of real or complex function values from <i>a</i> to <i>b</i> inclusive in integer steps</p>
+<p><b>summation( <i>f</i>, [<i>a</i>,<i>b</i>] )</b> &mdash; discrete summation of real or complex function values from <i>a</i> to <i>b</i> inclusive by unit step</p>
+
+<p><b>summation( <i>f</i>, [<i>a</i>,<i>b</i>,step] )</b> &mdash; discrete summation of real or complex function values from <i>a</i> to <i>b</i> inclusive by arbitrary step</p>
 
 <br id="interpolation"/>
 <h1>Interpolation</h1>
diff --git a/src/calculus/integration.js b/src/calculus/integration.js
index b04de26..ed8d5ba 100644
--- a/src/calculus/integration.js
+++ b/src/calculus/integration.js
@@ -241,13 +241,13 @@ function discreteIntegral( values, step ) {
 }
 
 
-function summation( f, [a,b] ) {
+function summation( f, [a,b,step=1] ) {
 
   if ( isComplex( f(a) ) ) {
 
     var s = complex(0);
 
-    for ( var i = a ; i <= b ; i++ ) s = add( s, f(i) );
+    for ( var i = a ; i <= b ; i += step ) s = add( s, f(i) );
 
     return s;
 
@@ -255,7 +255,7 @@ function summation( f, [a,b] ) {
 
     var s = 0;
 
-    for ( var i = a ; i <= b ; i++ ) s += f(i);
+    for ( var i = a ; i <= b ; i += step ) s += f(i);
 
     return s;