This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A087252 #16 Sep 27 2024 20:56:58 %S A087252 12,28,36,44,60,76,92,108,120,124,140,156,164,172,188,204,216,220,236, %T A087252 248,252,268,284,292,300,316,328,332,348,364,376,380,388,396,412,420, %U A087252 428,432,436,440,444,460,476,484,492,496,500,504,508,516,524,540,548 %N A087252 Numbers that are divisible by 4, but cannot be the largest peak value in a 3x+1 trajectory, regardless of the initial value. %C A087252 It is provable that (beyond 1 and 2) the largest peak value in any 3x+1 (Collatz) trajectory must be a multiple of 4. However, an infinite number of multiples of 4 exist that cannot be the largest peak value of such a trajectory. E.g., no integer of the form 16k+12 = 4*(4k+3) (where k is a nonnegative integer) can be a largest peak value, because the trajectory immediately after the value 16k+12 would consist of the values 8k+6, 4k+3, 12k+10, 6k+5, and 18k+16, which exceeds 16k+12. %H A087252 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a> %Y A087252 Cf. A025586. %K A087252 nonn %O A087252 1,1 %A A087252 _Labos Elemer_, Sep 08 2003 %E A087252 Definition and example reworded by _Jon E. Schoenfield_, Sep 01 2013