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In mathematics, the Mian–Chowla sequence is an integer sequence defined recursively in the following way. The sequence starts with

\( a_1 = 1. \)

Then for n>1, a_n is the smallest integer such that the pairwise sum

\( a_i + a_j \)

is distinct, for all i and j less than or equal to n.

Initially, with \( a_1 \), there is only one pairwise sum, 1 + 1 = 2. The next term in the sequence, a_2, is 2 since the pairwise sums then are 2, 3 and 4, i.e., they are distinct. Then, a_3 can't be 3 because there would be the non-distinct pairwise sums 1 + 3 = 2 + 2 = 4. We find then that a_3 = 4, with the pairwise sums being 2, 3, 4, 5, 6 and 8. The sequence thus begins

1, 2, 4, 8, 13, 21, 31, 45, 66, 81, 97, 123, 148, 182, 204, 252, 290, 361, 401, 475, ... (sequence A005282 in OEIS).

If we define \( a_1 = 0 \), the resulting sequence is the same except each term is one less (that is, 0, 1, 3, 7, 12, 20, 30, 44, 65, 80, 96, ... OEIS A025582).

The sequence was invented by Abdul Majid Mian and Sarvadaman Chowla.
References

S. R. Finch, Mathematical Constants, Cambridge (2003): Section 2.20.2
R. K. Guy Unsolved Problems in Number Theory, New York: Springer (2003)

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