A202025 - OEIS

%I #14 Oct 30 2018 10:31:02

%S 3,4,8,16,16,19,60,221,654,654,654,654,654,30291,30291,30291,30291,

%T 250231,342916,342916,472727,1934365,1934365,11877702,11877702,

%U 11877702

%N Position of second appearance of set of first n terms in the sequence of odd primes modulo 4.

%C Next term, a(27) > 3*10^7.

%e Consider the sequence of odd primes modulo 4: S= 3, 1, 3, 3, 1, 1, 3, 3, 1, 3, 1, 1, 3, 3, 1, 3, 1,... . Then

%e a(1)=3 because 2nd appearance of 3 is S(3),

%e a(2)=4 because 2nd appearance of (3,1) begins at S(4),

%e a(3)=8 because 2nd appearance of (3,1,3) begins at S(8),

%e a(4)=16 because 2nd appearance of (3,1,3,3) begins at S(16).

%t nn=3*10^7; s=Table[Mod[Prime[n],4], {n,2,nn}]; Reap[k1=2; Do[tn=Take[s,n]; Do[If[tn==Take[s,{k,k+n-1}], Sow[k]; k1=k; Break[]], {k,k1,nn-n-1}], {n,26}]][[2,1]]

%Y Cf. A039702.

%K nonn

%O 1,1

%A _Zak Seidov_, Dec 09 2011