diff --git a/hw4/assignment4.m b/hw4/assignment4.m
index c1e8c9c..0072451 100644
--- a/hw4/assignment4.m
+++ b/hw4/assignment4.m
@@ -1,7 +1,7 @@
 %% Assignment 4
 % Name: Claudio Maggioni
 %
-% Date: 19/3/2019
+% Date: 2020-05-17
 %
 % This is a template file for the first assignment to get started with running
 % and publishing code in Matlab.  Each problem has its own section (delineated
@@ -11,11 +11,13 @@
 % To generate a pdf for submission in your current directory, use the following
 % three lines of code at the command window:
 %
-%  >> options.format = 'pdf'; options.outputDir = pwd; publish('assignment3.m', options)
+%  >> options.format = 'pdf'; options.outputDir = pwd; publish('assignment4.m', options)
 %
 
 %% Problem 3
-clear;
+clear all;
+clc;
+clf reset;
 
 n=10;
 f = @(x) (exp(-(x^2)/2)/sqrt(2*pi));
@@ -36,12 +38,21 @@ p_10 = computePolypoints(f, xe, x_10, 10);
 p_5c = computePolypoints(f, xe, x_5c, 5);
 p_10c = computePolypoints(f, xe, x_10c, 10);
 
-figure;
-subplot(1,2,1);
 plot(xe, p_5, xe, p_10, xe, y);
-subplot(1,2,2);
+figure;
 plot(xe, p_5c, xe, p_10c, xe, y);
 
+%% Question 6
+
+y = [-0.0044; -0.0771; -0.2001; -0.3521; -0.3520; 0; 0.5741; 0.8673; ...
+    0.5741; 0; 0.3520; -0.3521; 0.2001; -0.0771; -0.0213; -0.0044];
+figure;
+alpha = b3interpolate(y);
+x = (0:0.01:16)';
+y_c = spline_curve(alpha, x);
+plot(x, y_c);
+
+%% Question 3 (continued)
 function x = computeEquidistantXs(n)
     x = zeros(2*n+1,1);
     for i = 1:2*n+1
@@ -88,3 +99,46 @@ function p = HornerNewton(N, x, xe)
         p = p .* (xe - x(i)) + N(i);
     end
 end
+
+%% Problem 6 (continued)
+
+% assuming x_i = i - 1
+function [alpha] = b3interpolate(y)
+    n = size(y, 1);
+    A = zeros(n+2);
+    B = [y; 0; 0];
+    for x_i = 0:n-1 
+        for j = -1:n
+            %fprintf("A(%d, %d) = B3(%d - %d)\n", x_i + 1, j+2, x_i, j);
+            A(x_i + 1, j + 2) = B3(x_i - j);
+        end
+    end
+    A(n+1, 1:3) = [1 -2 1];
+    A(n+2, n-1:n+1) = [1 -2 1];
+    alpha = A \ B;
+end
+
+function [v] = spline_curve(alpha, x)
+    n = size(alpha, 1) - 2;
+    v = zeros(size(x));
+    for i = 1:size(x,1)
+        for j = -1:n
+            v(i) = v(i) + alpha(j + 2) * B3(x(i) - j);
+        end
+    end
+end
+
+function [y] = B3(x)
+y = zeros (size (x));
+i1 = find (-2 < x & x < -1);
+i2 = find (-1 <= x & x < 1);
+i3 = find (1 <= x & x < 2);
+
+y(i1) = 0.5 * (x(i1) + 2) .^ 3;
+y(i2) = 0.5 * (3 * abs (x(i2)) .^ 3 - 6 * x(i2) .^ 2 + 4);
+y(i3) = 0.5 * (2 - x(i3)) .^ 3;
+y = y / 3;
+end
+ 
+
+
diff --git a/hw4/assignment4.pdf b/hw4/assignment4.pdf
new file mode 100644
index 0000000..b1929fb
Binary files /dev/null and b/hw4/assignment4.pdf differ
diff --git a/hw4/hw4.pdf b/hw4/hw4.pdf
index 376cbc4..581436e 100644
Binary files a/hw4/hw4.pdf and b/hw4/hw4.pdf differ
diff --git a/hw4/hw4.tex b/hw4/hw4.tex
index a2af726..d82a35c 100644
--- a/hw4/hw4.tex
+++ b/hw4/hw4.tex
@@ -184,7 +184,7 @@ a_0\\a_1\\a_2\\a_{-1}\\
 y_0\\y_1\\y_2\\y_{3}\\
 \end{bmatrix}\]
 
-\subsection*{Question 5}
+\section*{Question 5}
 
 $$1 = y_0 = s(x_0) = 0 = a_{-1}B_3(1) +a_0B_3(0)
 +a_1B_3(-1)