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 A078274 #17 Jun 26 2025 07:41:10
%S A078274 11,1331,153351,17355371,1937557391,11315977951311,113351719917151331,
%T A078274 1135517391111917351351,11375175911311113917551371,
%U A078274 113951779115111331115917751391,1231517991171115311351117917952311,12335271911911173115511371119927152331
%N A078274 a(n) is the number from which if odd-positioned digits are deleted then one gets the concatenation of first n odd numbers, while if even-positioned digits are deleted then one gets reverse concatenation of first n odd numbers. Position of least significant digit is considered to be 1.
%C A078274 a(n) is formed by interleaving the digits of A019519(n) and A038395(n). - _Sean A. Irvine_, Jun 26 2025
%e A078274 a(4) = 17355371: deleting alternate digits starting from the LSD gives 1357. Deleting the other digits gives 7531.
%o A078274 (PARI) a(n) = {my(d, v=w=[1]); for(i=2, n, v=concat(v, d=digits(2*i-1)); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2]))); } \\ _Sean A. Irvine_, Jun 26 2025
%Y A078274 Cf. A078275, A078276, A078277, A078278.
%Y A078274 Cf. A019519, A038395.
%K A078274 base,nonn
%O A078274 1,1
%A A078274 _Amarnath Murthy_, Nov 25 2002
%E A078274 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003
%E A078274 Corrected and extended by _Harvey P. Dale_, Apr 11 2012
%E A078274 Original terms restored by _Sean A. Irvine_, Jun 26 2025