OFFSET
4,1
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 4..10000
FORMULA
Straddle primes are the nearest primes preceding and following composite n.
EXAMPLE
The first straddle prime pair is 3 and 5 which straddles the composite number 4 and 3+5 = 8 the first entry in the table.
PROG
(PARI) strad(n) = my (y, pp, np); for(x=1, n, y=composite(x); pp=precprime(y); np=nextprime(y); print1(pp+np", "))
composite(n) = my(c=1, x=1); while(c <= n, x++; if(!isprime(x), c++)); x
(PARI) list(lim)=my(v=List(), p=3, t); forprime(q=5, , t=p+q; if(t>lim, return(Vec(v))); for(i=2, q-p, listput(v, t)); p=q) \\ Charles R Greathouse IV, Jan 22 2016
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Mar 13 2005
STATUS
approved