Let A={0,1,2,3,…} and F={f∣f:A→A}. The binary operation ⊙ on F is defined pointwise as (f1⊙f2)(n)=f1(n)+f2(n) for all n∈A.
Let us evaluate the algebraic properties of (F,⊙):
Closure:
For any f1,f2∈F and n∈A, both f1(n)∈A and f2(n)∈A. Since the sum of two non-negative integers is a non-negative integer, (f1⊙f2)(n)=f1(n)+f2(n)∈A. Hence, f1⊙f2∈F.
Associativity:
For any f1,f2,f3∈F and n∈A:
((f1⊙f2)⊙f3)(n)=(f1(n)+f2(n))+f3(n)=f1(n)+(f2(n)+f3(n))=(f1⊙(f2⊙f3))(n) Associativity holds because addition of integers is associative.
Commutativity:
For any f1,f2∈F and n∈A:
(f1⊙f2)(n)=f1(n)+f2(n)=f2(n)+f1(n)=(f2⊙f1)(n) Commutativity holds because integer addition is commutative.
Identity Element:
Define e:A→A by e(n)=0 for all n∈A. Since 0∈A, e∈F. For any f∈F:
(f⊙e)(n)=f(n)+e(n)=f(n)+0=f(n) Thus, e serves as the identity element under ⊙.
Inverses:
For an inverse g∈F of a function f∈F, we require (f⊙g)(n)=e(n)=0, meaning f(n)+g(n)=0 for all n∈A. Since f(n),g(n)≥0, this requires g(n)=−f(n). If f(n)>0 for any n, then g(n)<0∈/A, so g∈/F. Thus, inverses do not exist for all elements in F.
Since (F,⊙) is associative, commutative, has an identity element, but lacks inverses for all elements, (F,⊙) is an Abelian monoid.
Therefore, option B is correct.