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Numbers n such that the sum of the digits of the n-th lower twin prime is a lower twin prime.
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%I #10 Mar 14 2015 00:05:07

%S 1,5,11,13,14,15,16,18,19,24,29,31,32,33,35,36,40,43,48,52,55,57,62,

%T 63,64,69,71,74,77,80,82,84,85,86,88,94,97,98,101,106,109,111,112,115,

%U 116,117,118,124,125,130,132,135,138,140,143,145,151,154,157,160,162,163

%N Numbers n such that the sum of the digits of the n-th lower twin prime is a lower twin prime.

%e The 13th lower twin prime is 179. 1+7+9=17 the lower twin prime of the twin pair (17,19).

%o (PARI) sdtwtw(n) = { local(x,y); for(x=1,n, y=sumdigits(twinl(x)); if(istwin(y)<0, print1(x",") ) ) } sumdigits(n) = \ The sum of the digits of n { local(x,j,s=0); x=digits(n); for(j=1,length(x), s+=x[j]; ); return(s) } digits(n) = \ The vector of the digits of n { return(eval(Vec(Str(n)))) } twinl(n) = \The n-th lower twin prime { local(c,x); c=0; x=1; while(c<n, if(isprime(prime(x)+2),c++); x++; ); return(prime(x-1)) } istwin(n) = \Return -1 if n is a lower twin,1 if upper, otherwise return 0 { local(p1,p2); if(isprime(n), p1=n-2; p2=n+2; if(isprime(p1),return(1)); if(isprime(p2),return(-1)); return(0) ) }

%K easy,nonn,base

%O 1,2

%A _Cino Hilliard_, Mar 13 2005