A072492 - OEIS

A072492

Values of n for which A072491(n)=3.

27, 35, 51, 57, 65, 77, 87, 93, 95, 117, 119, 121, 122, 123, 125, 135, 143, 145, 147, 148, 155, 161, 171, 177, 185, 187, 189, 190, 203, 205, 207, 208, 209, 215, 217, 219, 220, 221, 237, 245, 247, 249, 250, 255, 261, 267, 275, 287, 289, 291, 292, 297, 299

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Define f(1) = 0. For n>=2, let f(n) = n - p where p is the largest prime <= n. A072491(n) = number of iterations of f to reach 0, starting from n.

p+4 is a term if p is a prime but p+2 and p+4 are both composite.

LINKS

Table of n, a(n) for n=1..53.

EXAMPLE

27 is a term as f(27)=27-23=4, f(4)=4-3=1 and f(1) = 0. (3 steps.)

MATHEMATICA

f[1]=0; f[n_] := n-Prime[PrimePi[n]]; a72491[n_] := Module[{k, x}, For[k=0; x=n, x>0, k++; x=f[x], Null]; k]; Select[Range[300], a72491[ # ]==3&]