Speci–c Examples, Generic Elements and Size Tuning - Tools...

Specific Examples, Generic Elements and Size Tuning -Tools for Overcoming Student Roadblocks in Linear

Algebra

Jeffrey L. StuartPacific Lutheran UniversityTacoma, WA 98447 USAjeffrey.stuart@plu.edu

January 2016

Jeffrey L. StuartPacific Lutheran UniversityTacoma, WA 98447 USAjeffrey.stuart@plu.edu ( )Specific Examples, Generic Elements and Size Tuning - Tools for Overcoming Student Roadblocks in Linear AlgebraJanuary 2016 1 / 12

My Background

Fifteen years at PLU - a medium-sized, liberal arts university

15 —20 math-related majors per year, all take linear algebra assophomores or juniors.Plus some physics, CS and econ majors, also some math/stats minorsWe offer a single, proof-based linear algebra courseHalf of students take an Intro to Proofs course first

Fifteen years at a medium-sized state university with a masters inmath and a joint PhD in Computational Science

15 - 20 math majors per year, most of whom took two semesters ofproof-based linear algebraWe also offered a low-proof matrix theory course for math ed, and twograduate computational matrix theory coursesHalf of students in first proof-based course took Discrete Math first

Thirty years as a researcher in combinatorial matrix theory

(Thank you, Professor Hans Schneider)

Student Problems in Proofs (1)

Linear Algebra is the often the first math course in which sets play anexplicit and fundamental role.Linear Algebra students typically struggle with writing proofs forset-based results.

Anyone who has taught linear algebra several times has seen student"proofs" of the following types:

Prove that vector addition is commutative in Rn.

Putative Proof: "(4, 3) and (5, 7) are in Rn, and(4, 3) + (5, 7) = (9, 10) = (5, 7) + (4, 3)."

A specific example is employed to prove the truth of a universalstatement, and a proof for R2 dispatches Rn for all n. Note thatspecifying R47 rather than Rn blocks student from using R2. Since47 is too large to explicitly write each entry, students are pushedtowards focusing on a general entry.

Prove that the set{(a, b) ∈ R2 : a ≥ 0 and b ≥ 0

}is closed under

vector addition."

Putative Proof: "(a, b) + (a, b) = (2a, 2b) satisfies 2a ≥ 0 and2b ≥ 0."The single generic element from the set definition used torepresent two potentially distinct generic elements.

This is a proof, but of not of the desired result.

}is closed under

vector addition."

Putative Proof: "(a, b) + (a, b) = (2a, 2b) satisfies 2a ≥ 0 and2b ≥ 0."

The single generic element from the set definition used torepresent two potentially distinct generic elements.

}is closed under

vector addition."

}is closed under

vector addition."

Prove that the set{(a, b) ∈ R2 : ab = 0

}is not closed under vector

addition.

Putative Proof: "(a1 + a2)(b1 + b2) = 0 soa1b1 + a2b1 + a1b2 + a2b2 = 0 so a2b1 + a1b2 = 0, which cannothappen."Except that it does sometimes happen. The student has failed toconvince us because the entries in the argument are not fullyspecified, allowing the possibility for a1 = a2 = 0 or b1 = b2 = 0.On the positive side, the student has created and correctly used twogeneric elements from the set, (a1, b1) and (a2, b2), and implicitlybut correctly used a1b1 = 0 and a2b2 = 0.

In the first "proof", a specific example "verifies" a universalstatement.In the second "proof", a generic example falsely contradicts a falseuniversal statement. The student fails to see the need for a specificexample.

addition.Putative Proof: "(a1 + a2)(b1 + b2) = 0 soa1b1 + a2b1 + a1b2 + a2b2 = 0 so a2b1 + a1b2 = 0, which cannothappen."

Except that it does sometimes happen. The student has failed toconvince us because the entries in the argument are not fullyspecified, allowing the possibility for a1 = a2 = 0 or b1 = b2 = 0.On the positive side, the student has created and correctly used twogeneric elements from the set, (a1, b1) and (a2, b2), and implicitlybut correctly used a1b1 = 0 and a2b2 = 0.

addition.Putative Proof: "(a1 + a2)(b1 + b2) = 0 soa1b1 + a2b1 + a1b2 + a2b2 = 0 so a2b1 + a1b2 = 0, which cannothappen."Except that it does sometimes happen. The student has failed toconvince us because the entries in the argument are not fullyspecified, allowing the possibility for a1 = a2 = 0 or b1 = b2 = 0.

On the positive side, the student has created and correctly used twogeneric elements from the set, (a1, b1) and (a2, b2), and implicitlybut correctly used a1b1 = 0 and a2b2 = 0.

addition.Putative Proof: "(a1 + a2)(b1 + b2) = 0 soa1b1 + a2b1 + a1b2 + a2b2 = 0 so a2b1 + a1b2 = 0, which cannothappen."Except that it does sometimes happen. The student has failed toconvince us because the entries in the argument are not fullyspecified, allowing the possibility for a1 = a2 = 0 or b1 = b2 = 0.On the positive side, the student has created and correctly used twogeneric elements from the set, (a1, b1) and (a2, b2), and implicitlybut correctly used a1b1 = 0 and a2b2 = 0.

In the first "proof", a specific example "verifies" a universalstatement.

In the second "proof", a generic example falsely contradicts a falseuniversal statement. The student fails to see the need for a specificexample.

Prompting for generic elements and specific examples

Students need constant re-emphasis of the differences between andthe roles of generic elements and specific (counter)examples.

"Can anyone give me two specific examples from this set?"

"Can anyone give me two generic elements from this set? Whatproperties do they satisfy?"

"We are proving a result for all of the elements in this set. What is aoften good way to begin?"

This (eventually) prompts students to volunteer, "Choose genericelements from the set."

"We want to show a property can fail for some elements in a set.What is a often good way to begin?"

This (eventually) prompts students to volunteer, "Look for a specificelement (counterexample) in the set."

Author/Instructor-Induced Student Problems in Proofs

Prove the sum of two upper triangular matrices is upper triangular.

Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.

The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.

Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.

Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.

Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)

The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.

Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Prove the sum of two upper triangular matrices is upper triangular.Hidden set question: Show that the set of n× n upper triangularmatrices is closed under matrix addition.The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (A+ B)ij = aij + bij . Since A and B are uppertriangular, aij = 0 and bij = 0 when i > j , so aij + bij = 0 wheni > j . Thus, A+ B is upper triangular.Putative Proof 1: The sum of two specific upper triangular matrices.Putative Proof 2: Partially filled, possibly upper triangular matricesmostly added together.Putative Proof 3: (Blank)The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.Most students can generate generic elements and perform the matrixalgebra to correctly show that the sum of two 3× 3 upper triangularmatrices is upper triangular.

Author/Instructor-Induced Misery (Aggravated)

Prove the product of two upper triangular matrices is upper triangular.

The Fantasy Proof: Let A and B be n× n upper triangular matrices.For all i and j , (AB)ij = ∑n

k=1 aikbkj . Since A and B are uppertriangular, aik = 0 when i > k and bkj = 0 when k > j , so aikbkj = 0when k < i or k > j . So (AB)ij = ∑j

k=i aikbkj . When i > j , there areno nonzero summands! Thus, (AB)ij = 0 when i > j .

I leave you to imagine the horrors that I have seen.

The arbitrary aspects overwhelm the students, concealing that theydo understand the importance of triangularity here.

Most students can generate the generic elements and perform thematrix algebra to correctly show that the product of two 3× 3 uppertriangular matrices is upper triangular.

Four Key Strategies for Student Proof Success

1 Emphasize the role of specific (fully specified) examples asexamples to highlight definitions, and, more importantly, ascounterexamples to universal statements.

2 Emphasize what a generic element from a set is, how to write oneor more generic elements from a set, and what role they play in proofsabout sets.

3 Emphasize the different and noninterchangeable roles of specificexamples and generic elements.

4 Thoughtfully tune the sizes of vectors and matrices in problems tofocus students on the primary idea at hand.

Spans are the Unsung Heroes of Linear Algebra ... but ...

"We" typically define the span of a set S of vectors from Rn as, "Thespan of S is the set of all linear combinations of vectors from S ."

Perhaps we require S be nonempty.

In all early examples, S is a FINITE set, but many authors/instructorsdo NOT build finiteness into their definition.

What does a student do with"Let S be a nonempty subset of Rn. Show that span(S) is closedunder vector addition?

Size tuning suggests a cascade of examples and problems.

If u, v,w ∈ Rn, thenspan(u, v,w) = {c1u+ c2v+ c3w : c1, c2, c3 ∈ R} .

Give a generic element of span(u, v,w).(Hint: It is not cu+ cv+ cw, why not?)Give a second, different generic element of span(u, v,w).Show that span(u, v,w) is closed under vector addition.

If v1, v2, . . . vk ∈ Rn, thenspan(v1, v2, . . . vk ) =

{∑kj=1 cjvj : all cj ∈ R

Give two different generic elements of span(v1, v2, . . . vk ).Show that span(v1, v2, . . . vk ) is closed under vector addition.

If S is a nonempty, possibly infinite subset of Rn, then span(S) is theset of all linear combinations built using a finite number of vectorsfrom S . To get a generic element of S , choose some positive integerk, choose k vectors from S , and then form the linear combination:x = ∑k

j=1 cjvj where v1, v2, . . . vk ∈ S and where c1, c2, . . . , ck ∈ R.

Why can we assume that another generic element y in S can be builtwith the same vectors v1, v2, . . . vk ∈ S ?Prove span(S) is closed under vector addition by showing x+ y is in S .

Give a generic element of span(u, v,w).(Hint: It is not cu+ cv+ cw, why not?)

Give a second, different generic element of span(u, v,w).Show that span(u, v,w) is closed under vector addition.

Give a generic element of span(u, v,w).(Hint: It is not cu+ cv+ cw, why not?)Give a second, different generic element of span(u, v,w).

Show that span(u, v,w) is closed under vector addition.

Give two different generic elements of span(v1, v2, . . . vk ).

Show that span(v1, v2, . . . vk ) is closed under vector addition.

j=1 cjvj where v1, v2, . . . vk ∈ S and where c1, c2, . . . , ck ∈ R.Why can we assume that another generic element y in S can be builtwith the same vectors v1, v2, . . . vk ∈ S ?

Prove span(S) is closed under vector addition by showing x+ y is in S .

j=1 cjvj where v1, v2, . . . vk ∈ S and where c1, c2, . . . , ck ∈ R.Why can we assume that another generic element y in S can be builtwith the same vectors v1, v2, . . . vk ∈ S ?Prove span(S) is closed under vector addition by showing x+ y is in S .

Mathematical conventions are agreements made by all or mostmathematicians to adopt a practice that is convenient.

You have met some conventions already: angles are measuredcounterclockwise; 0! = 1; and x0 = 1 for all real x .Another convention is that the span of the empty set is {0} where0 ∈ Rn for the appropriate choice of n. Does this seem reasonable toyou?

On the subject of conventions, I hope that you have enjoyed ourlovely Convention Center here in Seattle!

I wish to thank all of my students, the organizers of this session, andall you who have listened to this talk.

Jeff Stuart Pacific Lutheran University jeffrey.stuart@plu.edu

You have met some conventions already: angles are measuredcounterclockwise; 0! = 1; and x0 = 1 for all real x .

Another convention is that the span of the empty set is {0} where0 ∈ Rn for the appropriate choice of n. Does this seem reasonable toyou?

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