A205684 Numbers k for which 5 divides prime(k)-prime(j) for some j

4, 6, 7, 7, 9, 9, 10, 11, 12, 12, 12, 13, 13, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 18, 18, 18, 19, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 21, 22, 22, 22, 23, 23, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 27

Offset: 1

Author

Clark Kimberling, Jan 30 2012

Keywords

• nonn

Comments

For a guide to related sequences, see A205558.

Examples

			The first six terms match these differences:
p(4)-p(1)=7-2=5=5*1
p(6)-p(2)=13-3=10=5*2
p(7)-p(1)=17-2=15=5*3
p(7)-p(4)=17-7=10=5*2
p(9)-p(2)=23-3=20=5*4
p(9)-p(6)=23-13=10=5*2
		

Crossrefs

Cf. A205558, A204892, A204890, A205685, A205689.

Programs

Mathematica

s[n_] := s[n] = Prime[n]; z1 = 400; z2 = 80;
f[n_] := f[n] = Floor[(-1 + Sqrt[8 n - 7])/2];
Table[s[n], {n, 1, 30}]        (* A000040 *)
u[m_] := u[m] = Flatten[Table[s[k] - s[j], {k, 2, z1}, {j, 1, k - 1}]][[m]]
Table[u[m], {m, 1, z1}]        (* A204890 *)
v[n_, h_] := v[n, h] = If[IntegerQ[u[h]/n], h, 0]
w[n_] := w[n] = Table[v[n, h], {h, 1, z1}]
d[n_] := d[n] = Delete[w[n], Position[w[n], 0]]
c = 5; t = d[c]                (* A205683 *)
k[n_] := k[n] = Floor[(3 + Sqrt[8 t[[n]] - 1])/2]
j[n_] := j[n] = t[[n]] - f[t][[n]] (f[t[[n]]] + 1)/2
Table[k[n], {n, 1, z2}]        (* A205684 *)
Table[j[n], {n, 1, z2}]        (* A205685 *)
Table[s[k[n]], {n, 1, z2}]     (* A205686 *)
Table[s[j[n]], {n, 1, z2}]     (* A205687 *)
Table[s[k[n]] - s[j[n]], {n, 1, z2}]     (* A205688 *)
Table[(s[k[n]] - s[j[n]])/c, {n, 1, z2}] (* A205689 *)