9801 (nine thousand eight hundred [and] one) is the natural number following 9800 and preceding 9802.
In mathematics 9801 is most notable for being the third, and only 4-digit square pentagonal number, since 9801 is a square number, which can be written as: 99 2 {\displaystyle 99^{2}} , and it is also a pentagonal number, since it can also be written as: 81 ⋅ ( 3 ⋅ 81 − 1 ) 2 = 81 ⋅ 242 2 {\displaystyle {\frac {81\cdot (3\cdot 81-1)}{2}}={\frac {81\cdot 242}{2}}} . 9801 is also the number that is below 10,000 which are integral multiples of their reversals. The only other number which has this property is 8712. 9801 is the sixth number such as the binomial coefficient is a perfect square.
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