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A145709 Number of primes among first 10^n primes that have first and last digits both equal 1. 4

%I

%S 1,6,39,393,7737,103788,1230898,13344414,120133545,1092391851

%N Number of primes among first 10^n primes that have first and last digits both equal 1.

%e a(1)=1 because among first 10^1 primes (i.e., from 2 to 29) there is just one prime with first and last digits equal 1, that is 11.

%o (UBASIC) 10 'leading-trailing digits 20 N=11:C=4:Q=1 30 'print 2;3; 40 A=3:S=sqrt(N) 50 B=N\A 60 if B*A=N then 190 70 A=A+2 80 if A<=S then 50 90 R=str(N) 100 T=left(R,2):X=val(T) 110 U=right(R,1):Y=val(U) 120 if X=Y and X=1 then Aa=Aa+1 130 if X=Y and X=3 then Bb=Bb+1 140 if X=Y and X=7 then Cc=Cc+1 150 if X=Y and X=9 then Dd=Dd+1 160 C=C+1 170 'print C;N;X;Y;Aa;Bb;Cc;Dd 180 if C=10^Q then print C;N;Aa;Bb;Cc;Dd:Q=Q+1 190 N=N+2:goto 40

%Y A145710, A145711, A145712, A145713

%K more,nonn,base

%O 1,2

%A _Enoch Haga_, Oct 16 2008

%E Edited and two more terms added by _Max Alekseyev_, Sep 11 2009

%E a(9), a(10) from _D. S. McNeil_, Aug 29 2010

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Last modified June 6 18:59 EDT 2020. Contains 334832 sequences. (Running on oeis4.)