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Next: References Up: The Natural Numbers Previous: Comments on Hilbert's Program

Practice With Induction

  1. Prove the formula for the sum of the first n natural numbers:

    displaymath642

  2. Guess and prove a formula for the sum of the first n odd natural numbers.
  3. Prove the formula for the sum of the first n+1 terms of an arithmetic progression:

    displaymath643

    (Note that this equals the average of the first and last terms, multiplied by the number of terms.)

  4. Prove the formula for the sum of the first n+1 terms of a geometric progression:

    displaymath644

  5. Prove the formula for the sum of the squares of the first n natural numbers:

    displaymath645

  6. Prove the formula for the sum of the cubes of the first n natural numbers:

    displaymath646

  7. The Fibonacci numbers are defined by tex2html_wrap_inline678 and thereafter tex2html_wrap_inline680 , tex2html_wrap_inline682 . Prove the formula:

    displaymath647

  8. Prove that the set tex2html_wrap_inline684 has exactly tex2html_wrap_inline686 subsets.
  9. For nonnegative integers n and k, define tex2html_wrap_inline692 to be the number of subsets of size k contained in a set of size n. First prove (but not necessarily by induction!) that

    displaymath648

    Then prove that

    displaymath649

    remembering that we define 0!=1.

  10. Prove the binomial formula:

    displaymath650

  11. Find and prove a formula for the number of ways n different men can be completely paired up with n different women.
  12. A group of 2n individuals are to be paired off into n pairs to play checkers. Find and prove a formula for the number of ways to do this.
  13. Prove that the Tower of Hanoi puzzle with n disks can be solved in tex2html_wrap_inline710 moves.
  14. Find and prove a formula for the maximum number of regions a plane can be divided up into using exactly n lines.
  15. Find and prove a formula for the maximum number of regions space can be divided up into using exactly n planes.
  16. Observe that

    displaymath651

    Guess and prove the general result.

  17. Guess and prove a formula for

    displaymath652

    for tex2html_wrap_inline716 .

  18. Prove that every natural number greater than 1 can be factored into powers of primes.
  19. Define (a,b) to be the greatest common divisor of natural numbers a and b. Assume without loss of generality that tex2html_wrap_inline726 . Suppose c is the remainder when a is divided by b.
    1. Prove (without induction) that (b,c)=(a,b).
    2. Prove by induction (on what?) that if z=(a,b) then there exist integers m and n such that z=ma+nb.
    3. Prove (without induction) that if z is relatively prime to m and z is a divisor mn, then z is a divisor of n.
    4. Let n be a natural number greater than 1. Assume that tex2html_wrap_inline760 where tex2html_wrap_inline762 are primes and tex2html_wrap_inline764 are positive natural numbers. Assume also that tex2html_wrap_inline766 where tex2html_wrap_inline768 are primes and tex2html_wrap_inline770 are positive natural numbers. Prove by induction on n that tex2html_wrap_inline774 , tex2html_wrap_inline776 , tex2html_wrap_inline778 , and tex2html_wrap_inline780 , tex2html_wrap_inline778 .

  20. Prove that the number of ways to divide up a convex n-gon, tex2html_wrap_inline786 , into triangles using its diagonals is

    displaymath653

  21. Three students, A, B, and C are seated in a circle facing each other. There is a supply of three red hats and two black hats, and the students are aware of the types of hats available. The students close their eyes, and a hat is placed on each one's head. The unused two hats are hidden. When they open their eyes, each can see the hats of the other two, but cannot see their own hat. Unknown to them, each has been given a red hat. Each one is asked in turn, ``Do you know the color of your hat?'' A answers truthfully, ``No.'' B does the same. But C's answer is ``Yes, my hat is red.'' How did C know this? Generalize to n students.


next up previous
Next: References Up: The Natural Numbers Previous: Comments on Hilbert's Program

Carl Lee
Wed Sep 16 09:26:16 EDT 1998