A196788 a(n) is the first occurrence of n in A196697.

1, 1805, 133, 2, 3, 4, 5, 13, 6, 9, 8, 10, 19, 16, 32, 24, 74, 30, 18, 60, 168, 20, 42, 90, 180, 210, 186, 408, 144, 1020, 1050, 900, 2520, 3348, 2850, 5520, 3390, 774, 5760

Offset

1,2

Comments

a(39+k) > 9594, for any k >= 1. It is getting much harder for n>39. It took the Mathematica program weeks to get the first 39 items.

Links

Table of n, a(n) for n=1..39.

Example

A196697(1)=1, a(1)=1;
A196697(2)=4, a(4)=2;
...
A196697(1805)=2, a(2)=1805; (for any k<1805, A196697(k)<>2)

Mathematica

b = 2; max = 39; Array[fa, max]; Do[
fa[k] = 0, {k, 1, max}]; filled = 0; i = 0; While[filled < max, i++;
c1 = b^i; cs = {};
Do[c2 = b^j; cp = c1 + c2 + 1; If[PrimeQ[cp], cs = Union[cs, {cp}]];
  cp = c1 + c2 - 1; If[PrimeQ[cp], cs = Union[cs, {cp}]];
  cp = c1 - c2 + 1; If[PrimeQ[cp], cs = Union[cs, {cp}]];
  cp = c1 - c2 - 1;
  If[PrimeQ[cp], cs = Union[cs, {cp}]], {j, 0, i - 1}];
ct = Length[cs];
If[ct <= max, If[fa[ct] == 0, fa[ct] = i; filled++]]]; Table[
fa[k], {k, 1, max}]

Crossrefs

Cf. A196697.

Sequence in context: A252676 A248063 A256027 * A252029 A250385 A345765

Adjacent sequences: A196785 A196786 A196787 * A196789 A196790 A196791

Keyword

nonn,more

Author

Lei Zhou, Oct 06 2011

Status

approved