A104216 Primes p such that the sum of the digits of the p-th lower twin prime is a lower twin prime.

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

Links

Table of n, a(n) for n=1..54.

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

Adjacent sequences: A104213 A104214 A104215 * A104217 A104218 A104219

Keyword

easy,nonn,base

Author

Cino Hilliard, Mar 13 2005

Status

approved