A209266 - OEIS

%I #32 Mar 12 2013 04:31:33

%S 0,0,1,1,2,2,2,1,3,3,1,3,3,4,3,5,4,2,5,4,4,4,4,3,3,6,6,4,4,3,4,5,6,3,

%T 6,5,4,5,5,6,4,3,4,5,5,2,5,4,6,4,6,6,3,7,5,7,6,4,7,6,5,5,7,5,4,5,8,6,

%U 7,6,8,6,7,9,4,6,5,5,8,3,6,6,5,4,6,5,7,7,8

%N a(n) is the number of 3-prime arithmetic progression prime chains surrounding the n-th prime number with 5-smooth intervals

%C Based on the conjecture in A211376, a(n) > 0.

%C Last appearance of positive integers in a(n) at n<220000

%C a(11)=1 (a(n) > 1 for 11<n<220000, the same hereinafter);

%C a(46)=2; a(10680)=3; a(32293)=4; a(212493)=5

%H Lei Zhou, <a href="/A209266/b209266.txt">Table of n, a(n) for n = 1..10000</a>

%e n=3: prime(3)=5, 3,5,7 form a 3-prime arithmetic progression prime chain with the interval of 2, a 5-smooth number.  And this is the only case.  So a(3)=1;

%e ...

%e n=43: prime(43)=191, the following 3-prime arithmetic progression prime chains exists:

%e   149,191,233 (gap 42=2*3*7, not 5-smooth)

%e   131,191,251 (gap 60=2^2*3*5, 5-smooth)

%e   113,191,269 (gap 78=2*3*13, not 5-smooth)

%e   101,191,281 (gap 90=2*3^2*5, 5-smooth)

%e   89,191,293  (gap 102=2*3*17, not 5-smooth)

%e   71,191,311  (gap 120=2^3*3*5, 5-smooth)

%e   29,191,353  (gap 162=2*3^4, 5-smooth)

%e   23,191,359  (gap 168=2^3*3*7, not 5-smooth)

%e   3,191,379   (gap 188=2^2*47, not 5-smooth)

%e Among these groups, there are 4 5-smooth gaps.  So, a(43)=4.

%t Table[p = Prime[i]; ct = 0; Do[If[(PrimeQ[p - j]) && (PrimeQ[p + j]),

%t    f = Last[FactorInteger[j]][[1]]; If[f <= 5, ct++]], {j, 2, p,

%t    2}]; ct, {i, 3, 89}]

%Y Cf. A078611, A051037, A001031, A211376

%K nonn,easy

%O 1,5

%A _Lei Zhou_, Feb 07 2013