diff --git a/src/logarithmus.c b/src/logarithmus.c
index a7abbbb..8dbfd7a 100644
--- a/src/logarithmus.c
+++ b/src/logarithmus.c
@@ -5,7 +5,7 @@
 #include "exponent.h"
 
 double logX(double b, double a){
-    if(b == 1.0 || b < 0.0){
+    if(b == 1.0 || b <= 0.0 || a <= 0.0){
         return -1.0;
     }
     return(logN(a, 0.000001)/logN(b, 0.000001));
diff --git a/src/logarithmus.h b/src/logarithmus.h
index 4294276..68b011f 100644
--- a/src/logarithmus.h
+++ b/src/logarithmus.h
@@ -3,5 +3,7 @@
 
 double logN(double x, double eps);
 double logX(double b, double a);
+double logDec(double a);
+double logBin(double a);
 
 #endif // logarithmus.h
\ No newline at end of file
diff --git a/test/test_logarithmus.c b/test/test_logarithmus.c
index 6c056d8..e8fedfe 100644
--- a/test/test_logarithmus.c
+++ b/test/test_logarithmus.c
@@ -37,5 +37,24 @@ void test_ln_with_zero_and_negative_numbers(void){
     TEST_ASSERT_EQUAL_DOUBLE(-1.0, r3);
 }
 
+void test_logarithmic_function_with_edge_cases(void){
+    double r1, r2, r3, r4;
+
+    r1 = logX(0, 10);
+    r2 = logX(-1.0, 9);
+    r3 = logX(1, 18);
+    r4 = logX(3, 0);
+
+    printf("%lf\n", r1);
+    printf("%lf\n", r2);
+    printf("%lf\n", r3);
+    printf("%lf\n", r4);
+
+    TEST_ASSERT_EQUAL_DOUBLE(-1.0, r1);
+    TEST_ASSERT_EQUAL_DOUBLE(-1.0, r2);
+    TEST_ASSERT_EQUAL_DOUBLE(-1.0, r3);
+    TEST_ASSERT_EQUAL_DOUBLE(-1.0, r4);
+}
+
 
 #endif // TEST
\ No newline at end of file