OFFSET
1,3
EXAMPLE
The a(4) = 6 arithmetic progressions are:
(2), (2,3), (2,5), (3), (3,5), (5).
The a(5) = 11 arithmetic progressions are:
(2), (2,3), (2,5), (2,7), (3), (3,5), (3,5,7), (3,7), (5), (5,7), (7).
PROG
(PARI) apc(v)={#v + sum(i=1, #v-1, sum(j=i+1, #v, my(s=v[j]-v[i], t=v[j]+s); while(setsearch(v, t), t+=s); (t-v[j])/s))}
a(n)={apc(Set(primes(n-1)))} \\ Andrew Howroyd, Jan 29 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Giovanni Teofilatto, Jan 28 2007
EXTENSIONS
a(5) corrected and terms a(7) and beyond from Andrew Howroyd, Jan 29 2020
STATUS
approved