seven

 seven

What is seven?

In Mathematics, the short answer is :

 Seven is a natural number ,also a prime number; which is denoted by "7".

   But, this is not all about seven!

  Let's chat with seven...

    We know that seven is considered as a lucky number. It is also called a happy number, safe number...

   But , what Mathematics say about 7???

  Let's see...

    7 is the 4-th prime number. It is a Mersenne prime (as, 2³ -1 =7); not only that,  it is also a double Mersenne prime(as, the exponent 3 itself be a Mersenne prime).

 7 is a factorial prime. It is a Harshad number also.
    Let us consider the random experiment of rolling two standard die simultaneously.
 The occurrence of getting 7 as result is 6 times(1-6,6-1,2-5,5-2,3-4,4-3) out of 36(6×6) times. Thus the the probability of getting 7 is=6/36=1/6.
    The last digit of Graham's number is seven.
   Seven  is the only dimension, besides the familiar three , in which a vector cross product can be defined .
  A seven-sided shape is called a heptagon .
  Seven  is the lowest dimension of a known exotic sphere . But, there may exist as yet unknown exotic smooth structures on the four-dimensional sphere.

   The "Millennium Prize" Problems are seven problems in mathematics which were stated by the "Clay Mathematics Institute" in 2000. Currently, six of the problems remains unsolved .

  Now, let's see what world say about 7:

(1) There are seven days in a week.

 (2) There are seven seas/oceans in the world ( North Atlantic, South Atlantic, Arctic, North Pacific, South Pacific, Indian, Southern).

(3) There are seven continents in the world (Asia, Europe, North America, South America, Africa, Australia, Antarctica).

(4) seven classical planets (i.e., the seven moving objects in the sky visible in the naked eye) ( Mars, Jupiter, Venus, Saturn, Mercury, moon and the sun itself!).

(5) Seven colours in rainbow (VIBGYOR).

(6) There are seven basic  musical notes(Indian version: sa, re, ga , ma, pa , dha, ni. Western version: do, re, me, fa, so, la, te.).

(7) There are seven logic gates: 

  NOT, AND, OR, NOR, NAND,XOR, XNOR.

(8) There are seven rows in the periodic table.

(9) There are seven heveans.
(10)In China, the entire seventh month of the lunar calendar is considered the Ghost month.
      We will close this topic with an interesting story. Yes, there is an interesting story about seven.
     Seven men were accused of Christianity around the  250 AD , when the  Roman emperor Decius ruled. They took refuge in a cave and fell asleep.The emperor saw his chance to get rid of them once and for all and ordered the cave to be sealed.
Many decades later a farmer opened the cave and found the Seven Sleepers.
They woke up believing they had only slept a day.
In 1927 the “Gotto” near Ephesus was excavated.
The ruins of a church was found and on the walls inscriptions dedicated to the Seven Sleepers.

 If you find out any incorrect information or know anything more about this , please write it in the comment section!