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Sequence of the Day

A008619: (Positive integers repeated twice.)

{ 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, ... }
Smallest positive integer whose harmonic mean with another positive integer is . For example, is already given (as 4 is the smallest positive integer such that the harmonic mean of 4 (with 12) is 6) —but the harmonic mean of 2 (with − 6) is also 6 and 2 < 4, so the two positive integer restrictions need to be imposed to rule out both 2 and − 6.
Note also that this sequence is the second outermost diagonal of Lozanić’s triangle.

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Sequences in the news

Sequences in the News
 Dec 25 2018 German HeiseNews "integers, please" column explains A003173 and OEIS.
 Feb 01 2018 Alphabet announced a $8,589,869,056 = $A000396(6) stock buyback.
 Jan 03 2018 Largest known term of A000043 announced: 77232917.
 Nov 18 2016 PrimeGrid proves that 10223 is not a Sierpinski number, since 10223 × 2 31172165 + 1 is prime. So no changes to A076336 for now.
 Sep 14 2016 Tom Greer discovers the twin primes 2996863034895 × 2 1290000 ± 1 using PrimeGrid, TwinGen and LLR.
 Jan 19 2016 Largest known term of A000043 announced: 74207281, also discovered by Curtis Cooper.


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