In statistics, the lowercase formula_1 can be used to represent degrees of freedom. In physics, the same letter can also used to represent kinematic viscosity and the frequency of a wave.
Flag of Haiti
The flag of Haiti was adopted on February 25, 1986. The flag has two horizontal bands of equal height; the top stripe is blue and the bottom is red. In the center there is a white rectangle with the Coat of Arms of Haiti which consists of a palm tree with several flags at its sides. There are two cannons on either side of the palm tree. The civil flag and the ensign (the one used by commercial ships) do not have a coat of arms.
The Haitian flag was created by the revolutionary Jean-Jacques Dessalines in 1803 from the French flag (the "Tricolore"); he eliminated the white stripe and left only the blue and red stripes in the new flag.
From May 25 1964 to February 25 1986, during the dictatorship of Francois and Jean-Claude Duvalier, the country had a different flag, in red and black, and divided vertically, not horizontally.
Xi
Xi (uppercase/lowercase Ξ ξ) is the 14th letter of the Greek alphabet. It is used to represent the "x" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 60. The Roman letter X and Cyrillic letter Ѯ came from Xi.
In mathematics, both uppercase Ξ and lowercase ξ are used to refer to variants of Riemann Xi function (itself a variant of Riemann zeta function). In physics, the uppercase Ξ is used to represent the cascade particle and xi baryon.
Omicron
Omicron (uppercase/lowercase Ο ο) is a letter of the Greek alphabet. It is used to represent the "o" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 70. Letters that came from it include the Roman O and Cyrillic О.
In mathematics and computer science, the uppercase Ο is used in big-O notation, while the lowercase ο is used in little-o notation (a variant of big-O notation).
Pi (letter)
Pi (uppercase/lowercase Π π), is the letter of the Greek alphabet, used to represent the "p" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 80. Letters that came from it include the Roman P and Cyrillic П.
In mathematics, the uppercase pi is used for the Pi product operator (for example. formula_1), similar to the way summation is used. The lowercase pi is used as the mathematical constant pi, also known as Archimedes's constant.
San (letter)
San (uppercase/lowercase Ϻ ϻ) was an old letter in the Greek alphabet that was used in some Greek cities before the alphabet got its classical standard form. It looked like the letter Mu (M), but was spoken like S. In the 6th century BC, people stopped using it and wrote Sigma (Σ) instead.
Sho (letter)
Sho (uppercase/lowercase Ϸ ϸ), was a letter added to the Greek alphabet in order to write the Bactrian language. It was used for the "sh" sound in Bactrian.
Koppa (letter)
Koppa or Qoppa (uppercase/lowercase ) was a letter of the Greek alphabet that was used in some Greek cities before the alphabet got its classic standard form. It is no longer used in normal writing today. It was pronounced as "k", just like Kappa (Κ). In the 5th century BC, people stopped using it and it fell out of use. Kappa then replaced it. In the order of the letters in the alphabet, it came between Pi and Rho, just like in the Latin alphabet the letter Q comes between P and R. However, they kept Koppa as a sign for the number "90" in the system of Greek numerals, but in a third numerical form. People still sometimes use it in this way, even though they do not use it as a letter to write words.
The letter Koppa at first looked like a circle with a small bar under it. Later it was written more like a pointed S.
The letter Q in the Latin alphabet and the letter Ҁ in the Cyrillic alphabet come from Koppa.
Rho
Rho (uppercase/lowercase Ρ ρ) is the 17th letter of the Greek alphabet. It is used to represent the "r" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 100. Letters that came from it include the Roman R and Cyrillic Р.
In statistics, the lowercase Rho ("ρ") is used to represent population correlation. In physics, the same letter is used to represent density.
Sigma
Sigma (uppercase/lowercase Σ σ ς) is the 18th letter of the Greek alphabet. It is used to represent the "s" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 200. Letters that came from it include the Roman S and Cyrillic С.
The uppercase Σ is the shorthand symbol for summation (the symbol is also known as the summation operator). It is often used to condense Taylor series and to write out trigonometric functions such as sine, cosine, and tangent as series.
The lowercase σ is used to represent population standard deviation in statistics, and permutation function in abstract algebra.
Tau
Tau (uppercase/lowercase Τ τ), is the letter of the Greek alphabet, used to represent the "t" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 300. Letters that came from it include the Roman T and Cyrillic Т.
Some people want to use Tau in place of Pi, the special number that comes from circles. Tau would be equal to two times pi, or about 6.283.
These people want to use Tau because they think it would be easier for everyone to understand. There are many reasons in math why Tau could be useful or helpful. One of the simplest is that it would make learning about radians easier.
A radian is a way of measuring the angle of a circle. It says that the number of radians in an angle is equal to the length of the outside of the circle, or the 'arc length', that is covered by an angle, divided by the radius, or distance from the point in the center of a circle to anywhere on the outside (see the picture on the bottom right of the page).
If the distance covered around the outside of a circle is equal to the radius, than one radian is being covered. Around the whole outside of a circle, there are about 6.283 radians - or, Tau radians (Tau is just a number, like 0, or 7, or 100).
The advantage of this is that one quarter of a circle would simply become Tau / 4 radians, and one half of a circle Tau / 2 radians, which is more in line with our intuition on circles (see also the drawing below).
Not many people use Tau yet, but more and more are.
Upsilon
Upsilon (uppercase/lowercase Υ υ) is the 20th letter of the Greek alphabet. It is used to represent the "u" sound in Ancient Greek and "i" in Modern Greek. In the system of Greek numerals, it has a value of 400. Letters that came from it include the Roman U, V and Y, and the Cyrillic У.
In mathematics, the uppercase formula_1 is used to represent the upsilon function (a variant of Riemann zeta function), while the lowercase formula_2 is used as a general variable. In physics, the uppercase formula_1 is used to represent an upsilon meson. In astrology, the same letter is used for the sign of Aries.
Phi
Phi (uppercase/lowercase Φ φ), is the 21st letter of the Greek alphabet, used to represent the "ph" sound in Ancient Greek. This sound changed to "f" some time in the 1st century AD, and in Modern Greek the letter denotes the "f" sound. In the system of Greek numerals, it has a value of 500. The Cyrillic letter Ф came from Phi. In English, φ is pronounced like "f" but transliterated (re-written) as "ph" in words which originate in Ancient Greek. In those words which originate in Modern Greek, such as feta cheese, φ is transliterated as "f". The letter Phi is used to represent the golden ratio (which is about 1.618).
Chi (letter)
Chi (uppercase/lowercase Χ χ) is the 22nd letter of the Greek alphabet. It is used to represent the "ch" sound (as in Scottish "loch" or German "Bauch") in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 600. Letters that came from it include the Roman X and Cyrillic Х.