A104215 Numbers n such that the sum of the digits of the n-th lower twin prime is a lower twin prime.

1, 5, 11, 13, 14, 15, 16, 18, 19, 24, 29, 31, 32, 33, 35, 36, 40, 43, 48, 52, 55, 57, 62, 63, 64, 69, 71, 74, 77, 80, 82, 84, 85, 86, 88, 94, 97, 98, 101, 106, 109, 111, 112, 115, 116, 117, 118, 124, 125, 130, 132, 135, 138, 140, 143, 145, 151, 154, 157, 160, 162, 163

Offset

1,2

Links

Table of n, a(n) for n=1..62.

Example

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

Prog

(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) ) }

Crossrefs

Sequence in context: A176821 A070369 A357995 * A287123 A227146 A251965

Adjacent sequences: A104212 A104213 A104214 * A104216 A104217 A104218

Keyword

easy,nonn,base

Author

Cino Hilliard, Mar 13 2005

Status

approved