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 A261348 #27 Dec 31 2020 11:11:15 %S A261348 0,0,1,1,2,1,2,2,2,2,3,2,3,3,3,3,4,3,4,4,4,4,5,4,5,5,5,5,6,5,6,6,6,6, %T A261348 7,6,7,7,7,7,8,7,8,8,8,8,9,8,9,9,9,9,10,9,10,10,10,10,11,10,11,11,11, %U A261348 11,12,11,12,12,12,12,13,12,13,13,13,13,14,13,14,14,14,14,15,14,15,15,15,15,16,15,16,16,16,16,17,16 %N A261348 a(1)=0; a(2)=0; for n>2: a(n) = A237591(n,2) = A237593(n,2). %C A261348 n is an odd prime if and only if a(n) = 1 + a(n-1) and A237591(n,k) = A237591(n-1,k) for the values of k distinct of 2. %C A261348 For k > 1 there are five numbers k in the sequence. %C A261348 For more information see A237593. %e A261348 Apart from the initial two zeros the sequence can be written as an array T(j,k) with 6 columns, where row j is [j, j, j+1, j, j+1, j+1], as shown below: %e A261348 1, 1, 2, 1, 2, 2; %e A261348 2, 2, 3, 2, 3, 3; %e A261348 3, 3, 4, 3, 4, 4; %e A261348 4, 4, 5, 4, 5, 5; %e A261348 5, 5, 6, 5, 6, 6; %e A261348 6, 6, 7, 6, 7, 7; %e A261348 7, 7, 8, 7, 8, 8; %e A261348 8, 8, 9, 8, 9, 9; %e A261348 9, 9, 10, 9, 10, 10; %e A261348 10, 10, 11, 10, 11, 11; %e A261348 11, 11, 12, 11, 12, 12; %e A261348 12, 12, 13, 12, 13, 13; %e A261348 13, 13, 14, 13, 14, 14; %e A261348 14, 14, 15, 14, 15, 15; %e A261348 15, 15, 16, 15, 16, 16; %e A261348 ... %e A261348 Illustration of initial terms: %e A261348 Row _ %e A261348 1 _| |0 %e A261348 2 _| _|0 %e A261348 3 _| |1| %e A261348 4 _| _|1| %e A261348 5 _| |2 _| %e A261348 6 _| _|1| | %e A261348 7 _| |2 | | %e A261348 8 _| _|2 _| | %e A261348 9 _| |2 | _| %e A261348 10 _| _|2 | | | %e A261348 11 _| |3 _| | | %e A261348 12 _| _|2 | | | %e A261348 13 _| |3 | _| | %e A261348 14 _| _|3 _| | _| %e A261348 15 _| |3 | | | | %e A261348 16 _| _|3 | | | | %e A261348 17 _| |4 _| _| | | %e A261348 18 _| _|3 | | | | %e A261348 19 _| |4 | | _| | %e A261348 20 _| _|4 _| | | _| %e A261348 21 _| |4 | _| | | | %e A261348 22 _| _|4 | | | | | %e A261348 23 _| |5 _| | | | | %e A261348 24 _| _|4 | | _| | | %e A261348 25 _| |5 | _| | | | %e A261348 26 | |5 | | | | | %e A261348 ... %e A261348 The figure represents the triangle A237591 in which the numbers of horizontal cells in the second geometric region gives this sequence, for n > 2. %e A261348 Note that this is also the second geometric region in the front view of the stepped pyramid described in A245092. For more information see also A237593. %Y A261348 Cf. A000040, A236104, A235791, A237048, A237591, A237593, A261350, A261699. %K A261348 nonn,tabf,easy %O A261348 1,5 %A A261348 _Omar E. Pol_, Aug 24 2015