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 A139258 #5 Jun 29 2025 18:27:16 %S A139258 5,55,575,5775,57775,577775,5772775,57722775,577212775,5772112775, %T A139258 57721512775,577215512775,5772156512775,57721566512775, %U A139258 577215666512775,5772156666512775,57721566466512775,577215664466512775 %N A139258 Palindromes formed from the reflected decimal expansion of Euler's constant (or Euler-Mascheroni constant) gamma. %e A139258 n ... Successive digits of a(n) %e A139258 1 ............ ( 5 ) %e A139258 2 ......... . ( 5 5 ) %e A139258 3 .......... ( 5 7 5 ) %e A139258 4 ......... ( 5 7 7 5 ) %e A139258 5 ........ ( 5 7 7 7 5 ) %e A139258 6 ....... ( 5 7 7 7 7 5 ) %e A139258 7 ...... ( 5 7 7 2 7 7 5 ) %e A139258 8 ..... ( 5 7 7 2 2 7 7 5 ) %e A139258 9 .... ( 5 7 7 2 1 2 7 7 5 ) %e A139258 10 .. ( 5 7 7 2 1 1 2 7 7 5 ) %t A139258 a[n_]:=FromDigits[Join[RealDigits[EulerGamma,10,Ceiling[n/2]][[1]],Reverse[RealDigits[EulerGamma,10,Floor[n/2]][[1]]]]];Array[a,18] (* _James C. McMahon_, Jun 29 2025 *) %Y A139258 Decimal expansion of gamma: A001620. Cf. A135634, A135697, A135700, A139259, A139260, A139261. %K A139258 easy,nonn,base %O A139258 1,1 %A A139258 _Omar E. Pol_, May 01 2008