%I A126805
%S A126805 1,1,1,1,2,1,1,1,3,2,2,1,2,2,4,2,3,2,3,2,1,2,3,3,1,2,2,3,1,2,2,2,2,4,2,
%T A126805 2,2,1,4,3,4,2,2,1,2,3,2,2,3,2,3,2,2,2,1,3,4,2,4,2,5,2,2,3,2,3,3,2,4,3,
%U A126805 3,5,3,3,2,3,2,3,2,2,3,2,3,1,2,2,2,2,4,3,4,3,1,2,4,3,3,2,3,2,2,5,3,3,2
%N A126805 "Class-" (or "class-minus") number of prime(n) according to the Erdos-Selfridge 
               classification of primes.
%C A126805 This gives the "class-" number as opposed to the "class+" number. Not 
               to be confused with the "class-number" of quadratic form theory.
%C A126805 a(n)=1 if A000040(n) is in A005109, a(n)=2 if A000040(n) is in A005110, 
               a(n)=3 if A000040(n) is in A005111 etc.
%F A126805 a(n) = max { a(p)+1 ; prime(p) is > 3 and divides prime(n)-1 } union 
               { 1 } - M. F. Hasler (Maximilian.Hasler(AT)gmail.com), Apr 16 2007
%p A126805 a := proc(n) option remember; local p,pf,e,res; if isprime(n) then pf 
               := ifactors(n-1)[2]; res := 1; for e from 1 to nops(pf) do p := op(1,
               op(e,pf)); if p > 3 then res := max(res,a(p)+1); fi; od; RETURN(res); 
               else -1; fi; end: for n from 1 to 180 do printf("%d, ",a(ithprime(n))); 
               end:
%o A126805 (PARI) A126805(n) = { if( n>0, n=-prime(n)); if(( n=factor(-1-n)[,1] 
               ) & n[ #n]>3, vecsort( vector( #n, i, A126805(-n[i]) ))[ #n]+1, 1) 
               } - M. F. Hasler (Maximilian.Hasler(AT)gmail.com), Apr 16 2007
%Y A126805 Cf. A005109, A005110, A005111, A005112, A081424, A081425.
%Y A126805 Cf. A081640, A081641, A129248, A056637.
%Y A126805 Sequence in context: A127832 A107249 A062842 this_sequence A110963 A106348 
               A161092
%Y A126805 Adjacent sequences: A126802 A126803 A126804 this_sequence A126806 A126807 
               A126808
%K A126805 easy,nonn
%O A126805 1,5
%A A126805 R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 23 2007