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 A077186 #12 May 25 2025 16:07:53
%S A077186 0,12321,0,12345654321,121,12345654321
%N A077186 Smallest multiple of prime(n) of the form 123...(k-1) k (k-1)...321 ( a concatenation of natural numbers from 1 to k and back to 1), or 0 if no such number exists.
%C A077186 a(7) has 435 digits (k=109) and is too large to include, a(8) has 399 digits (k=103), k(a(10))=1006, k(a(11))=17, k(a(12))=3 so a(12)=12321, k(a(13))=5 so a(13)=123454321, k(a(14))=103, k(a(17))=1019, k(a(18))=1002, k(a(19))=1001, k(a(20))=16. - _R. J. Mathar_, May 06 2006
%o A077186 (PARI) palid(n)= { local(resul) ; resul=concat("",n) ; forstep(i=n-1,1,-1, resul=concat(i,resul) ; resul=concat(resul,i) ; ) ; return(eval(resul)) ; }
%o A077186 A077186(n)= { local(p) ; if(n==1 || n==3, return(0) ; ) ; p=prime(n) ; for(i=1,1500, if( palid(i)%p ==0, return(i) ; break ; ) ; ) ; return(-1) ; }
%o A077186 for(n=1,20, print("n=",n," k=",A077186(n)) ; ) ; \\ _R. J. Mathar_, May 06 2006
%Y A077186 Cf. A077180, A077181, A077182, A077183, A077184, A077187, A077188, A173426.
%K A077186 base,nonn
%O A077186 1,2
%A A077186 _Amarnath Murthy_, Nov 01 2002
%E A077186 More terms from _R. J. Mathar_, May 06 2006