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 A342815 #22 Aug 31 2021 16:48:01
%S A342815 3,13,53,213,227,853,909,3413,3637,13653,14549,14563,54613,58197,
%T A342815 58253,218453,232789,233013,873813,931157,932053,932067,3495253,
%U A342815 3724629,3728213,3728269,13981013,14898517,14912853,14913077,55924053,59594069,59651413,59652309
%N A342815 Numbers of the form (2^(2*j + 6*k + 5) - 2^(2*j + 1) - 3)/9, with j,k >= 0.
%C A342815 Sequence is a subsequence of A198584. When any term is iterated using the Collatz function, the last odd integer in the trajectory before 1 is of the form (4^(3*m + 2) - 1)/3.
%H A342815 Satya Das, <a href="https://www.researchgate.net/publication/354253993_3X1_PROBLEM_A_CONTINUOUS_EXTENSION_OF_THE_SPEEDED_UP_COLLATZ_MAP">Extension of speeded up Collatz map to the real line</a>
%t A342815 Take[Sort[Flatten[Table[(2^(2n1+6n2+5) - 2^(2n1+1) - 3)/9, {n1, 0, 20}, {n2, 0, 20}]]], 50]
%o A342815 (Python)
%o A342815 seq=[]
%o A342815 for n1 in range(20):
%o A342815 for n2 in range(20):
%o A342815 n=(2**(2*n1+6*n2+5) - 2**(2*n1+1) - 3)/9
%o A342815 seq.append(n)
%o A342815 seq.sort()
%o A342815 print(seq[0:50])
%Y A342815 Union with A342816 gives A198584.
%K A342815 nonn,easy
%O A342815 1,1
%A A342815 _Satya Das_, Mar 22 2021