diff --git a/isabelle/2021/HSV_tasks_2021.thy b/isabelle/2021/HSV_tasks_2021.thy
new file mode 100644
index 0000000..2e56627
--- /dev/null
+++ b/isabelle/2021/HSV_tasks_2021.thy
@@ -0,0 +1,179 @@
+theory HSV_tasks_2021 imports Complex_Main begin
+
+section \<open>Task 1: Factorising circuits.\<close>
+
+(* Datatype for representing simple circuits. *)
+datatype "circuit" = 
+  NOT "circuit"
+| AND "circuit" "circuit"
+| OR "circuit" "circuit"
+| TRUE
+| FALSE
+| INPUT "int"
+
+(* Simulates a circuit given a valuation for each input wire. *)
+fun simulate where
+  "simulate (AND c1 c2) \<rho> = ((simulate c1 \<rho>) \<and> (simulate c2 \<rho>))"
+| "simulate (OR c1 c2) \<rho> = ((simulate c1 \<rho>) \<or> (simulate c2 \<rho>))"
+| "simulate (NOT c) \<rho> = (\<not> (simulate c \<rho>))"
+| "simulate TRUE \<rho> = True"
+| "simulate FALSE \<rho> = False"
+| "simulate (INPUT i) \<rho> = \<rho> i"
+
+(* Equivalence between circuits. *)
+fun circuits_equiv (infix "\<sim>" 50) (* the "50" indicates the operator precedence *) where
+  "c1 \<sim> c2 = (\<forall>\<rho>. simulate c1 \<rho> = simulate c2 \<rho>)"
+
+(* An optimisation that exploits the following Boolean identities:
+  `(a | b) & (a | c) = a | (b & c)`
+  `(a | b) & (c | a) = a | (b & c)`
+  `(a | b) & (b | c) = b | (a & c)`
+  `(a | b) & (c | b) = b | (a & c)`
+ *)
+fun factorise where
+  "factorise (NOT c) = NOT (factorise c)"
+| "factorise (AND (OR c1 c2) (OR c3 c4)) = (
+    let c1' = factorise c1; c2' = factorise c2; c3' = factorise c3; c4' = factorise c4 in
+    if c1' = c3' then OR c1' (AND c2' c4') 
+    else if c1' = c4' then OR c1' (AND c2' c3') 
+    else if c2' = c3' then OR c2' (AND c1' c4') 
+    else if c2' = c4' then OR c2' (AND c1' c3') 
+    else AND (OR c1' c2') (OR c3' c4'))"
+| "factorise (AND c1 c2) = AND (factorise c1) (factorise c2)"
+| "factorise (OR c1 c2) = OR (factorise c1) (factorise c2)"
+| "factorise TRUE = TRUE"
+| "factorise FALSE = FALSE"
+| "factorise (INPUT i) = INPUT i"
+
+lemma (* test case *)
+ "factorise (AND TRUE TRUE) = AND TRUE TRUE"
+  by eval
+lemma (* test case *)
+  "factorise (AND (OR (INPUT 1) FALSE) (OR TRUE (INPUT 1))) = 
+  OR (INPUT 1) (AND FALSE TRUE)"
+  by eval
+lemma (* test case *)
+  "factorise (NOT (AND (OR FALSE (INPUT 2)) (OR TRUE (INPUT 2)))) =
+  NOT (OR (INPUT 2) (AND FALSE TRUE))"
+  by eval
+
+theorem factorise_is_sound: "factorise c \<sim> c"
+  sorry
+
+fun factorise2 where "factorise2 c = c" (* dummy definition *)
+
+lemma (* test case *)
+  "factorise2 (OR (AND (INPUT 1) (INPUT 2)) (AND TRUE (INPUT 1))) = 
+  AND (INPUT 1) (OR (INPUT 2) TRUE)"
+  sorry
+
+theorem factorise2_is_sound: "factorise2 c \<sim> c"
+  sorry
+
+section \<open>Task 2: A theorem about divisibility.\<close>
+
+(* NB: Without the "::int" annotation, Isabelle will try to prove a 
+   slightly more general theorem where "a" and "b" can be either ints
+   or nats. That more general theorem is a little harder to prove. *)
+theorem plus_dvd_odd_power:
+  "(a::int) + b dvd a ^ (2 * n + 1) + b ^ (2 * n + 1)"
+  sorry
+
+
+section \<open>Task 3: Proving that the shift-and-add-3 algorithm is correct.\<close>
+
+subsection \<open>Binary and its conversion to nat\<close>
+
+type_synonym bit = "bool"
+
+abbreviation B0 where "B0 == False"
+abbreviation B1 where "B1 == True"
+
+
+fun binary_to_nat :: "bit list \<Rightarrow> nat"
+where
+  "binary_to_nat [] = 0"
+| "binary_to_nat (b # bs) = (if b then 2 ^ length bs else 0) + binary_to_nat bs"
+
+
+lemma (* test case *) "binary_to_nat [B0, B1, B0, B1] = 5" by eval
+lemma (* test case *) "binary_to_nat [B0, B0, B1, B0, B1] = 5" by eval
+lemma (* test case *) "binary_to_nat [B1] = 1" by eval
+lemma (* test case *) "binary_to_nat [B0] = 0" by eval
+
+subsection \<open>BCD and its conversion to nat\<close>
+
+type_synonym nibble = "bit * bit * bit * bit"
+
+fun nibble_to_nat :: "nibble \<Rightarrow> nat"
+where
+  "nibble_to_nat (B0,B0,B0,B0) = 0"
+| "nibble_to_nat (B0,B0,B0,B1) = 1"
+| "nibble_to_nat (B0,B0,B1,B0) = 2"
+| "nibble_to_nat (B0,B0,B1,B1) = 3"
+| "nibble_to_nat (B0,B1,B0,B0) = 4"
+| "nibble_to_nat (B0,B1,B0,B1) = 5"
+| "nibble_to_nat (B0,B1,B1,B0) = 6"
+| "nibble_to_nat (B0,B1,B1,B1) = 7"
+| "nibble_to_nat (B1,B0,B0,B0) = 8"
+| "nibble_to_nat (B1,B0,B0,B1) = 9"
+| "nibble_to_nat (B1,B0,B1,B0) = 10"
+| "nibble_to_nat (B1,B0,B1,B1) = 11"
+| "nibble_to_nat (B1,B1,B0,B0) = 12"
+| "nibble_to_nat (B1,B1,B0,B1) = 13"
+| "nibble_to_nat (B1,B1,B1,B0) = 14"
+| "nibble_to_nat (B1,B1,B1,B1) = 15"
+
+fun bcd_to_nat :: "nibble list \<Rightarrow> nat"
+where
+  "bcd_to_nat [] = 0"
+| "bcd_to_nat (n # ns) = bcd_to_nat ns + nibble_to_nat n * 10 ^ length ns"
+
+lemma (* test case *) "bcd_to_nat [(B0,B1,B1,B0)] = 6" by eval
+lemma (* test case *) "bcd_to_nat [(B0,B1,B1,B0),(B1,B0,B0,B1)] = 69" by eval
+lemma (* test case *) "bcd_to_nat [(B0,B0,B0,B0),(B1,B0,B0,B1)] = 9" by eval
+lemma (* test case *) "bcd_to_nat [(B0,B0,B1,B1),(B0,B0,B0,B0)] = 30" by eval
+
+
+subsection \<open>Converting binary to BCD\<close>
+
+fun binary_to_bcd :: "bit list \<Rightarrow> nibble list"
+where
+  "binary_to_bcd bs = []" (* dummy definition *)  
+
+lemma (* test case *)
+  "binary_to_bcd [B1,B0,B1,B0,B1,B0,B1] = [(B1,B0,B0,B0), (B0,B1,B0,B1)]" 
+  sorry
+
+subsection \<open>Checking that nibbles correspond to valid BCD digits\<close>
+
+fun valid_nibble :: "nibble \<Rightarrow> bool"
+where
+  "valid_nibble (B0,B0,B0,B0) = True"
+| "valid_nibble (B0,B0,B0,B1) = True"
+| "valid_nibble (B0,B0,B1,B0) = True"
+| "valid_nibble (B0,B0,B1,B1) = True"
+| "valid_nibble (B0,B1,B0,B0) = True"
+| "valid_nibble (B0,B1,B0,B1) = True"
+| "valid_nibble (B0,B1,B1,B0) = True"
+| "valid_nibble (B0,B1,B1,B1) = True"
+| "valid_nibble (B1,B0,B0,B0) = True"
+| "valid_nibble (B1,B0,B0,B1) = True"
+| "valid_nibble (B1,B0,B1,B0) = False"
+| "valid_nibble (B1,B0,B1,B1) = False"
+| "valid_nibble (B1,B1,B0,B0) = False"
+| "valid_nibble (B1,B1,B0,B1) = False"
+| "valid_nibble (B1,B1,B1,B0) = False"
+| "valid_nibble (B1,B1,B1,B1) = False"
+
+theorem binary_to_bcd_valid:
+  "list_all valid_nibble (binary_to_bcd bs)"
+  sorry
+  
+subsection \<open>Proof that the binary_to_bcd translation is correct.\<close>
+
+theorem binary_to_bcd_correct:
+  "bcd_to_nat (binary_to_bcd bs) = binary_to_nat bs"
+  sorry
+
+end
\ No newline at end of file
diff --git a/isabelle/2021/isabelle_exercises_2021.pdf b/isabelle/2021/isabelle_exercises_2021.pdf
new file mode 100644
index 0000000..0d66045
Binary files /dev/null and b/isabelle/2021/isabelle_exercises_2021.pdf differ