OFFSET
1,2
FORMULA
Generate the prime sequence with primes labeled 1 mod 4 or 3 mod 4. Add count of primes to sequence if just one prime occurs before interruption by a prime congruent to 1 mod 4.
EXAMPLE
a(3)=28 because 28 single primes occur below 10^3, each interrupted in the run by a prime congruent to 1 mod 4.
MATHEMATICA
A002144 = Join[{0}, Select[4 Range[0, 10^4] + 1, PrimeQ[#] &]];
A002145 = Select[4 Range[0, 10^4] + 3, PrimeQ[#] &];
lst = {}; Do[If[Length[s = Select[A002145, Between[{A002144[[i]], A002144[[i + 1]]}]]] == 1, AppendTo[lst, Last[s]]], {i, Length[A002144] - 1}]; Table[Count[lst, x_ /; x < 10^n], {n, 4}] (* Robert Price, May 31 2019 *)
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Enoch Haga, Mar 02 2004
EXTENSIONS
a(9)-a(11) from Chai Wah Wu, Mar 18 2018
STATUS
approved