21 (number)

21 (twenty-one) is the natural number following 20 and preceding 22.

In mathematics

21 is:

21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with the abundant square prime 18 being the first such member. Twenty-one is the first number to be the aliquot sum of three numbers 18, 51, 91.

21st Kohat Mountain Battery (Frontier Force)

The 21st Kohat Mountain Battery (Frontier Force) was an artillery unit of the British Indian Army. It was raised in 1851 as the No. 2 Horse or Punjab Light Field Battery, Punjab Irregular Force. It became the 21st Kohat Mountain Battery (Frontier Force) in 1903. In 1947, it was transferred to the Pakistan Army, where it exists as the 2nd Royal Kohat Battery (Frontier Force) of The First (SP) Medium Regiment Artillery (Frontier Force).

History

The 21st Kohat Mountain Battery was raised at Bannu by Lieutenant H Hammond on 1 February 1851 from horse artillery detachments of the Lahore Durbar, as part of the Punjab Irregular Force (Piffer). The Piffers were a collection of regular units accreted for expeditionary operations in the Punjab Frontier and Afghanistan. 21st Kohat Mountain Battery was the first unit to be formally raised as a permanent part of the Punjab Irregular Force, later designated as the Punjab Frontier Force. Is the senior most unit of the Frontier Force Regiment. After 1903 reforms, the Punjab Frontier Force began to ceased to be an expeditionary force and incorporated permanent units.