Let be any well-defined function.
We want to express it as a sum of an even function and an odd function.
Let us define two other functions as follows:
Claim I: F(x) is an even function.
Proof I; Since by definition , so so that F(x) is indeed an even function.
Claim 2: G(x) is an odd function.
Proof 2: Since by definition , so so that G(x) is indeed an odd function.
Proof 3: indeed.
I) Prove that every function can be represented as a sum of an even function and an odd function.
II)Let A, B, C be subsets of a set S. Prove the following statements and illustrate them with Venn Diagrams:
2a) The famous DeMorgan’s laws in their basic forms: and . Assume that both sets A and B are subsets of Set S. In words, the first is: union of complements is the complement of intersection; the second is: intersection of two complements is the complement of the union of the two sets.
Let us say that we need to prove: .
Proof: It must be shown that the two sets have the same elements; in other words, that each element of the set on LHS is an element of the set on RHS and vice-versa.
If , then and . This means that , and and . Since and , hence . Hence, .
Conversely, if , then and . Therefore, and . Thus, and , so that . QED.
III) Prove that if I and S are sets and if for each , we have , then .
It must be shown that each element of the set on the LHS is an element of the set on RHS, and vice-versa.
If , then and . Therefore, , for at least one . Thus, , so that .
Conversely, if , then for some , we have . Thus, and . Since , we have . Therefore, . QED.
IV) If A, B and C are sets, show that :
V) Let I be a nonempty set and for each let be a set. Prove that
5a) for any set B, we have :
5b) if each is a subset of a given set S, then
VI) Prove that if , , and are functions, then :
VII) Let be a function, let A and B be subsets of X, and let C and D be subsets of Y. Prove that:
7i) ; in words, image of union of two sets is the union of two images;
7ii) ; in words, image of intersection of two sets is a subset of the intersection of the two images;
7iii) ; in words, the inverse image of the union of two sets is the union of the images of the two sets.
7iv) ; in words, the inverse image of intersection of two sets is intersection of the two inverse images.
7v) ; in words, the inverse of the image of a set contains the set itself.
7vi) ; in words, the image of an inverse image of a set is a subset of that set.
For questions 8 and 9, we can assume that the function f is and a set A lies in domain X and a set C lies in co-domain Y.
8) Prove that a function f is 1-1 if and only if for all ; in words, a function sends different inputs to different outputs iff a set in its domain is the same as the inverse of the image of that set itself.
9) Prove that a function f is onto if and only if for all ; in words, the image of a domain is equal to whole co-domain (which is same as range) iff a set in its domain is the same as the image of the inverse image of that set.
Thanks Dr. Gowers’. These are invaluable insights into basics. Thanks for giving so much of your time.
In the Feb 23 2018 blog problem, we posed the following question:
Sum the following infinite series:
The sum can be written as:
, where .
Thus, . This is the answer.
If you think deeper, this needs some discussion about rearrangements of infinite series also. For the time, we consider it outside our scope.