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 A087600 #18 Jul 29 2024 06:18:25
%S A087600 6,78,666,7503,82621,828828,7552441,87311505,557362578,9901692450,
%T A087600 88893307128,934624072410,9836548472766,99245275962778,
%U A087600 994337011743076,5535761776004778,89253915287999385,865474782199906830,9888742361454004621
%N A087600 Largest n-digit term of A087597, or 0 if no such number exists.
%C A087600 Conjecture: No term is zero.
%e A087600 a(4) = 7503, A040115(7503) = 253 is triangular.
%o A087600 (PARI) dd(k)={ local(kshf,res,dig,odig,p) ; kshf=k ; res=0 ; odig=kshf % 10 ; p=0 ; while(kshf>9, kshf=floor(kshf/10) ; dig=kshf % 10 ; res += 10^p*abs(dig-odig) ; odig=dig ; p++ ; ) ; return(res) ; } isA000217(n)={ if( issquare(1+8*n), return(1), return(0) ) ; } A000217(n)={ return(n*(n+1)/2) ; } ndigs(n)={ local(nshft,res) ; res=0 ; nshft=n; while(nshft>0, res++ ; nshft=floor(nshft/10) ; ) ; return(res) ; } isA087597(n)={ if( isA000217(n) && isA000217(dd(n)), return(1), return(0) ) ; } A087600(n)={ local(k,T) ; k=ceil(-0.5+sqrt(0.25+2*10^n)) ; T=A000217(k) ; if(ndigs(T)>n, k-- ) ; while(1, T=A000217(k) ; if(ndigs(T)<n, return(0) ) ; if( isA087597(T), return(T) ) ; k-- ; ) ; } { for(n=2,21, print1(A087600(n),",") ; ) ; } \\ _R. J. Mathar_, Nov 19 2006
%Y A087600 Cf. A000217, A087597, A087598, A087599.
%K A087600 base,nonn
%O A087600 1,1
%A A087600 _Amarnath Murthy_, Sep 18 2003
%E A087600 More terms from _R. J. Mathar_, Nov 19 2006
%E A087600 a(16)-a(18) from _Donovan Johnson_, Jul 28 2010
%E A087600 a(19) from _Donovan Johnson_, Jun 19 2011
%E A087600 a(1)=6 prepended by _Max Alekseyev_, Jul 27 2024