diff --git a/ex1C_summation_of_specified_numbers/special_sum.h b/ex1C_summation_of_specified_numbers/special_sum.h
index 49d579a..a77964b 100644
--- a/ex1C_summation_of_specified_numbers/special_sum.h
+++ b/ex1C_summation_of_specified_numbers/special_sum.h
@@ -1,5 +1,18 @@
 #include <cstddef>
 
+/**
+  This function returns the sum of all positive integers less or equal n which are a multiples of 3 or of 5, WITH using a loop.
+  @param[in]    n         
+  @param[out]   M        
+*/
 size_t special_sum_loop(size_t n);
 
+
+/**
+  This function returns the sum of all positive integers less or equal n which are a multiples of 3 or of 5, WITHOUT using a loop.
+  Example: For n=15, we have 60 = 3+5+6+9+10+12+15 = (1+2+3+4+5)*3 + (1+2+3)*5 - 1*15
+  Formula: M = (\sum_{i=1}^{k_3} i)*3 + (\sum_{i=1}^{k_5} i)*5 - (\sum_{i=1}^{k_15} i)*15
+  @param[in]    n         
+  @param[out]   M        
+*/
 size_t special_sum_noloop(size_t n);
\ No newline at end of file