olymmath

Putnam 2015 B2

  • Session B

Given a list of the positive integers 1,2,3,4,…1,2,3,4,\dots, take the first three numbers 1,2,31,2,3 and their sum 66 and cross all four numbers off the list. Repeat with the three smallest remaining numbers 4,5,74,5,7 and their sum 1616. Continue in this way, crossing off the three smallest remaining numbers and their sum, and consider the sequence of sums produced: 6,16,27,36,…6, 16, 27, 36, \dots. Prove or disprove that there is some number in the sequence whose base 10 representation ends with 20152015.

Curated text Kedlaya Putnam archive. Quoted with source.

Solutions

Human solution Solution 1 by Manjul Bhargava, Kiran Kedlaya and Lenny Ng

The terms of the source let olymmath link to this solution but not copy it. Read the solution (Kedlaya Putnam archive).

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