 Sample puzzles of Martin Gardner, with solutions  The Guardian
 Martin Gardner's best math puzzles  part 1 (The Math Dude)
 Math questions (solvable online) for class 10 from NSTE
 International Mathematical Olympiad  Question paper archive
 Problem Solving Strategies  Arthur Engel  archive.org
 Entertaining Math Puzzles, by Martin Gardner
Amazing universe
Exploring the wonders of science
Sunday, May 13, 2018
Mathematical puzzles online  Links to books and blogs
Friday, April 17, 2015
Angular dimension of the sun as viewed from earth
Approximate calculation of the angle subtended by the sun when viewed from earth:Diameter of sun, x = 1392000 km
Distance of sun from earth, y = 149000000
Approximate angle = 2 tan^{1}((x / 2) / y) = 2 x 0.26763471480399391313430123468864 degrees = 0.53526942960798782626860246937729 degrees
Friday, February 8, 2013
The science behind meditation  is it brain training?
An interesting article on Matthieu Ricard, a researcher turned monk, suggests that meditation might act as a brain training exercise, helping the human mind to achieve superior levels of happiness and decreased propensity for negativity. One hopes more research is done in this area so that the exact mechanism by which meditation acts to bring peace of mind is understood, thereby helping the whole of mankind lead a more content life.
Thursday, February 7, 2013
Converting twips to pixels and viceversa
Here is a Ruby program to convert between twips and pixels:
#!/bin/ruby
# As per PostScript definition
TWIPS_PER_INCH = 1440
def twips_to_pixels(twips, dpi, round = true)
raise "Invalid DPI" if (dpi <= 0)
raise "Invalid twips" if (twips < 0)
num_pixels = ((twips / TWIPS_PER_INCH.to_f) * dpi)
return round ? num_pixels.round : num_pixels.floor
end
def pixels_to_twips(pixels, dpi, round = true)
raise "Invalid DPI" if (dpi <= 0)
raise "Invalid number of pixels" if (pixels < 0)
twips = ((pixels / dpi.to_f) * TWIPS_PER_INCH)
return round ? twips.round : twips.floor
end
def numeric?(str)
Float(str) != nil rescue false
end
def print_usage_and_exit
puts "Usage: #{$0} [t twips  p pixels] [d dpi]"
exit(1)
end
if __FILE__ == $0
print_usage_and_exit if ARGV.length != 4
dpi = nil
twips = nil
pixels = nil
[0, 2].each do option_index
option = ARGV[option_index]
option_val = ARGV[option_index + 1]
print_usage_and_exit unless (numeric?(option_val))
case option
when "d" then dpi = option_val.to_f
when "t" then twips = option_val.to_f
when "p" then pixels = option_val.to_f
else print_usage_and_exit
end
end
print_usage_and_exit if ((pixels.nil? && twips.nil?)  dpi.nil?)
res = twips.nil? ? pixels_to_twips(pixels, dpi) : twips_to_pixels(twips, dpi)
puts res
end
# As per PostScript definition
TWIPS_PER_INCH = 1440
def twips_to_pixels(twips, dpi, round = true)
raise "Invalid DPI" if (dpi <= 0)
raise "Invalid twips" if (twips < 0)
num_pixels = ((twips / TWIPS_PER_INCH.to_f) * dpi)
return round ? num_pixels.round : num_pixels.floor
end
def pixels_to_twips(pixels, dpi, round = true)
raise "Invalid DPI" if (dpi <= 0)
raise "Invalid number of pixels" if (pixels < 0)
twips = ((pixels / dpi.to_f) * TWIPS_PER_INCH)
return round ? twips.round : twips.floor
end
def numeric?(str)
Float(str) != nil rescue false
end
def print_usage_and_exit
puts "Usage: #{$0} [t twips  p pixels] [d dpi]"
exit(1)
end
if __FILE__ == $0
print_usage_and_exit if ARGV.length != 4
dpi = nil
twips = nil
pixels = nil
[0, 2].each do option_index
option = ARGV[option_index]
option_val = ARGV[option_index + 1]
print_usage_and_exit unless (numeric?(option_val))
case option
when "d" then dpi = option_val.to_f
when "t" then twips = option_val.to_f
when "p" then pixels = option_val.to_f
else print_usage_and_exit
end
end
print_usage_and_exit if ((pixels.nil? && twips.nil?)  dpi.nil?)
res = twips.nil? ? pixels_to_twips(pixels, dpi) : twips_to_pixels(twips, dpi)
puts res
end
Sunday, February 3, 2013
1/1 matrix  problem #29 from Problem solving strategies
Here is the solution to one of the problems mentioned in the earlier blog post:
Problem 29:
Suppose contrary to the claim a1 + b1 + … + a25 + b25 = 0. Then:
(a1 + a2 + … + a25) =
 (b1 + … + b25) ⋯ ①
Now note that ai= 1,if number of elements having value 1 in row i is
even, and 1 otherwise.
Thus (a1 + a2 + … + a25) =
x + y, where x is the number of rows
having odd number of 1s and y is the number of rows having even number of 1s ⋯ ②
Similarly, (b1 + …
+ b25) = m + n, where
m is
the number of columns
having
odd number of 1s and n
is
the number of columns
having
even number of 1s
⋯ ③
Combining ①, ② and ③ results in the
equation (y – x ) = (m  n) ⋯ ④
But (y + x) = 25, the total number of
rows in the matrix, and (m + n) = 25, the total number of columns in the
matrix. Thus (y + x) = (m + n) ⋯ ⑤
Combining ④ and ⑤ results in the equality
y = m, which implies that the number of rows with even number of 1s equals the
number of columns with odd number of 1s.
Define z = number of
1s in the matrix.
Suppose y is odd. Then, x must be even
and by original definition of x and y, z must be even (even number of rows with
odd number of 1s plus odd number of rows with even number of 1s, resulting in
even + even). If y is even, z must be odd. Thus parity(z) ≠ parity(y) ⋯⑥
On the other hand, by definition of m and
n, it can be shown that if m is odd, then z must be odd and if m is even then z
must
be even.
Thus parity(z) = parity(m). But this contradicts ⑥ since y = m as per an earlier equation.
Thus the assumption a1 + b1 + …
+ a25 + b25 = 0 is wrong.
Thursday, January 31, 2013
2 problems  Matrix numbering and integer colouring
Here are 2 interesting problems from chapter 2 of Arthur Engel's book:
1. The positive integers are colored black and white. The sum of two differently colored
numbers is black, and their product is white. What is the product of two white
numbers? Find all such colorings.
numbers is black, and their product is white. What is the product of two white
numbers? Find all such colorings.
2. Each element of a 25 × 25 matrix is either +1 or −1. Let ai be the product of all
elements of the ith row and bj be the product of all elements of the jth column.
Prove that a_{1} + b_{1} + · · · + a_{25} + b_{25} ≠ 0.
elements of the ith row and bj be the product of all elements of the jth column.
Prove that a_{1} + b_{1} + · · · + a_{25} + b_{25} ≠ 0.
Sunday, January 27, 2013
Tile colouring problem (#8) from Problem solving strategies
Problem: Prove that an a × b rectangle can be covered by 1 × n rectangles iff na or nb.
Solution: The proof is given by means of the following 2 claims.
Claim 1:
Given a
rectangular area is covered by tiles of dimension 1xn placed in horizontal or
vertical orientation,
it is
possible to remove the
tiles
one at a time in such a way that at any point of time, the covered region’s top
border consists of a sequence of steps increasing from left to right as shown
in diagram 1.
Proof:
Define step
blocks to
be rectangular regions defined by the steplike shape of the covered area. In
diagram 1, step
blocks are
represented by the areas shaded blue. Start from the left most step block and
consider the top left tile in that block. This tile must fall entirely within the current
block (call it block A) or must extend beyond the
right of the current
step block
(it cannot beyond the bottom of the current block since the first block is also
the bottommost). In the former
case, removing the
tile
will leave behind a step shape,
thereby proving the claim. In the latter case, inspect the next step block for
the possibility of removing a tile. The top left tile of this next block (call
it block B) cannot extend beyond the bottom of that block since the block A’s
top left tile occupies that portion as per the previous statement. Hence the
top left tile of block B too must either lie entirely within B or extend beyond
the right of B. As before, in the former case, removing the tile will leave a
step shape whereas in the latter case, the search for a tile to remove can be
repeated with the block to the right of C. Note that this procedure can be
repeated at most till the rightmost block is
reached, in which case the top left tile cannot extend beyond the right of the
block (and must therefore lie entirely within that block), thereby becoming
eligible for removal while retaining the step shape.
Claim 2: When
tiles are removed as per the procedure of claim 1 and neither a not b are
divisible by n, then at any point of time at least one of the step regions on
the top border will have a height and a width (as defined by diagram 2) that are
both indivisible by n.
Proof: The
proof is by induction on r, the number of tiles removed so far under the
procedure of claim 1. When r is 0, the covered region and also the only step
region is the original axb
rectangle. The height and width, namely a and b, are both indivisible by n as
per the assumption. Suppose the claim is true for values of r upto R.
Now consider r = (R + 1). Suppose the tile being removed at the current
iteration lies in a step region whose width is W and height Is H. If either W
or H is divisible by n, then there must be some other step region with width
and height indivisible by n and that region will remain after iteration (R +
1) as well. But if say W and H are both
indivisible by n. After removing the tile in this iteration, the 2 new step
regions would have dimensions (a) (W, H  1) and (W  n, H) or (b) (W, H
 n) and (W  1, H), depending on the orientation of the tile; in either case,
one of the new step regions would have its width and height indivisible by n.
Hence proved.
Diagram 1: Step
blocks denoted
by shaded blue regions

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