Sherlock Holmes’ mysterious 221

221B Baker Street happens to be the house of Sherlock Holmes. Incidentally, the number 221 has several significances.

221 = 13×17 is a product of two prime numbers. It is called a semi-prime number. Some other examples are 6(2×3), 10(2×5), 14(2×7), 21(3×7), 22(2×11), 26(2×13), 33(3×11), 35(5×7), 39(3×13), 49(7×7), 51(3×17), 55(5×11), 57(3×19), 62(2×31), 65(5×13), 69(3×23), 77(7×11), 82(2×41), 85(5×17), 86(2×43), 87(3×29), 91(7×13), etc.

‘221’ has another interesting feature: it can be expressed as the sum of five consecutive (37 + 41 + 43 + 47 + 53) as well as nine consecutive (11 + 13 + 17 + 19 + 23 + 29 + 31 + 37 + 41) prime numbers.