Indus Valley Civilizatoin:
Indus Valley civilization (Harappa civilization) was located in the northwestern part of ancient India (Today’s Eastern part of Pakistan and North Western part of India). This civilization at the banks of Indus valley was present around in BC 3500 and ended around in BC 1500 and is considered one of the earliest civilizations along with Mesopotamia and Egyptian civilization and biggest civilization of that time.
This civilization extended from Afghanistan to Pakistan to India till Gujarat. Lately excavated Dwarka, the hometown of Lord Krishna, which was/is submerged in Arabian sea long before 3500 BC might have been part of Indus valley civilization.
Harappans used decimal numeral system without zero and place value system.
Mathematics was used for practical purposes in measurement of weights & distances, scaling, fixed proportion adopted to build bricks, etc.
The decline of Indus valley civilization is very controversial topic. For many years Aryan invasion theory was followed which stated that the a tribe called Aryans came from Southwest Asia and Europe and invaded the civilization, killed many, remaining were made to flee towards south India thus bringing the caste system between white and blacks; Aryans and Dravidians. But later this theory was ruled out on the basis of various factors. Like flooding over large part of Gujarat (which submerged large part of Dwarka in Arabian Sea), sudden decline of the civilization is now believed to be because changes in climatic conditions which killed many people and very few managed to flee to central and eastern India.
Bricks which were baked under the sun or in fire and followed fixed proportion of 4:2:1 to maintain the stability of brick structure for constructions of walls in buildings.
To prepare bricks they had prepared a scale whose base unit was considered as 1.32 inches (3.35 cm).
They followed the weight system. 28 grams was considered as the base unit and accordingly they used more weights which were in ratio 1/20, 1/10, 1/5, 1/2, 1, 2, 5, 10, 20, 50, 100, 200, and 500 of base unit weight.
They had produced weights and vessels for storing water, grains and other items which were in shape of regular geometrical figures like barrels, cones, cylinders denoting basic knowledge of geometry.
Although Harappans didn’t had much mathematics as compared to Babylonians but it was the start of great era of Indian mathematics and the decimal numeral system of India which was spread to Arabs to Europe and finally to entire world is considered as evolved from this civilization.
Decline of civilization:
It is very unclear the exact cause of decline of Indus valley civilization.
Many theories like Aryan Invasion theory, Flooding, dependency only on monsoon for water, lack of irrigation system, Draughts, spread of epidemics, Tectonics, etc are considered as reason for sudden end to the civilization.