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A104216
Primes p such that the sum of the digits of the p-th lower twin prime is a lower twin prime.
0
5, 11, 13, 19, 29, 31, 43, 71, 97, 101, 109, 151, 157, 163, 191, 197, 211, 223, 227, 239, 241, 269, 281, 283, 347, 359, 401, 409, 421, 433, 443, 503, 541, 571, 617, 643, 677, 691, 727, 733, 739, 751, 773, 787, 839, 877, 907, 919, 929, 937, 953, 971, 977, 991
OFFSET
1,1
EXAMPLE
The 13th lower twin prime is 179. 1+7+9=17 the lower twin prime of the twin pair (17,19).
PROG
(PARI) sdtwtwnp(n) = { local(x, y); forprime(x=1, n, y=sumdigits(twinl(x)); if(istwin(y)<0, print1(x", ") ) ) }
sumdigits(n) = { local(x, j, s=0); x=digits(n); for(j=1, length(x), s+=x[j]; ); return(s) }
digits(n) = { return(eval(Vec(Str(n)))) }
twinl(n) = { 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) ) }
CROSSREFS
Cf. A001359.
Sequence in context: A206581 A292940 A098085 * A269819 A040144 A019395
KEYWORD
easy,nonn,base
AUTHOR
Cino Hilliard, Mar 13 2005
STATUS
approved