Pi is the fixed number you get when you divide a circle's circumference by its diameter, and this ratio is the same for any circle, no matter its size. Pi is the symbol for the ratio of a circle's circumference to its diameter, roughly equal to 3.1415926535.
Phi is the twenty-first letter of the Greek alphabet, pronounced 'fài'. It has an uppercase Φ and a lowercase φ. The top left corner of the lowercase is usually open, and sometimes it’s written as a smaller version of the uppercase. Besides being a letter, it also represents various specialized meanings in different fields.
Arabic numerals are actually the ten number symbols we use every day, from 0 to 9. Even though the name has 'Arabic' in it, they were actually invented by the ancient Indians and later got this name when the Arabs brought them to Europe. They are the number system used worldwide today.
Eratosthenes (around 275–194 BC) was a famous ancient Greek mathematician, astronomer, and geographer, born in Cyrene, Libya. He's best known for coming up with the first method to measure the Earth's circumference, getting an estimate with less than 5% error, and for being the first to use the term 'geography.' Because of this, he’s often honored as the 'Father of Geography.'
The Sieve of Eratosthenes is an ancient algorithm for quickly finding all prime numbers within a certain range, proposed by the ancient Greek mathematician Eratosthenes.
Albert Einstein was one of the greatest physicists of the 20th century, came up with the theory of relativity and the energy-mass equation, and won the 1921 Nobel Prize in Physics.
The "bitwise method" usually refers to "bitwise operations," which are low-level calculations that perform logic or shift operations on the binary bits of integers. It's not an independent "counting system" or "number base" (the latter is often mistakenly called "bitwise counting method").