Writing A Number As Sum Of Two Or More Consecutive Positive Integers - Kamal Lohia


  • Faculty and Content Developer at Tathagat | Delhi College of Engineering


    In how many ways can 2010 be written as sum of two or more than two consecutive positive integers?

    For example: 2010 = 669 + 670 + 671.

    Now every number can be written as a sum of consecutive integers.

    Ex (-3)+(-2)+(-1)+ 0 + 1+ 2+ 3+ 4 = 4

    But not all numbers can be written as sum of consecutive positive integers.

    A number can be written as the sum of consecutive positive integers if, and only if, its prime factorization includes an odd prime factor; then, since 2 is the only even prime number, the conclusion is that the powers of 2 are the only numbers that cannot be written as the sum of consecutive positive integers.
     

    Number of ways to write a natural number, N as sum of two or more than two consecutive positive integers is given by number of odd positive integral divisors of N - 1.

    So in this particular question, number of odd divisors of 2010 is 8. hence the required number of ways = 7.

    Now take an example of 36.

    now 36-1=35 has four positive odd integral divisors. So 36 can be expressed as sum of two or more positive consecutive integers in four possible ways. But how to construe those ways if the number is large?

    Ex 60; 60-1=59; 59 has two odd divisors, but the number of ways to express 60 as sum of consecutive positive integers is three:

    60 = 19+20+21 = 10+11+12+13+14 = 4+5+6+7+8+9+10+11

    Also in how many ways can any natural number be expressed as sum of consecutive positive even integers or consecutive possible odd integers?

    Number of ways to write a natural number,N as sum of two or more than two consecutive natural numbers = (number of odd divisors of N) - 1.
    If N = 36, then the required number of ways are = 3 - 1 = 2.
    If N = 60, then required number of ways are = 4 - 1 = 3.

    Now if question is to find the number of ways to write a natural number,N as sum of two or more than two consecutive even natural numbers, then first condition is that n must be an even number. Number of ways remains same as above. Let's check for 60.

    60 = 3*20 = 5*12 = 4*15
    So, 60 = 18 + 20 + 22 = 8 + 10 + 12 + 14 + 16 = 12 + 14 + 16 + 18.

    Now if question is to find number of ways to write a natural number,N as sum of two or more than two consecutive odd natural numbers, then N has to be an odd number or a multiple of 4. Number of ways can be calculated accordingly.


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