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A073754 Number of steps needed to reach a prime when the following map is repeatedly applied to n: if n is even then 2n + #(n) + 1, otherwise 2n - #(n) - 1, where #(n) is the number of digits of n; or -1 is no prime is ever reached. 5
3, 3, 2, 12, 2, 10, 11, 3, 1, 1, 9, 1, 1, 9, 2, 1, 10, 2, 1, 10, 1, 1, 8, 1, 2, 8, 1, 2, 13, 1, 1, 13, 1, 1, 9, 1, 1, 9, 1, 1, 35, 1, 2, 35, 8, 2, 7, 8, 1, 7, 1, 1, 52, 1, 6, 52, 21, 6, 12, 21, 1, 12, 1, 1, 12, 1, 1, 12, 28, 1, 8, 28, 1, 8, 5, 1, 6, 5, 1, 6, 1, 1, 34, 1, 5, 34, 1, 5, 71, 1, 7 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

LINKS

Table of n, a(n) for n=2..92.

EXAMPLE

For n=9, a(9)=3 because 9 -> 16 -> 35 -> 67

MATHEMATICA

pa[n_] := If[EvenQ[n], 2*n + IntegerLength[n] + 1, 2*n - IntegerLength[n] - 1]; Table[c = 0; If[PrimeQ[n], n = pa[n]; c = 1]; While[! PrimeQ[n] && c < 1000, n = pa[n]; c += 1]; If[c == 1000, c = -1]; c, {n, 2, 92}] (* Jayanta Basu, Jul 09 2013 *)

PROG

(UBASIC)

10 cls

30 for I=2 to 100

32 H=I

40 if odd(H)=1 then goto 90 else goto 50

50 A=2*H+alen(H)+1:K=K+1

60 if prmdiv(A)=A then print I, K:goto 120

65 if K>1000 then print I, 0:goto 120

70 H=A:goto 40

90 A=2*H-alen(H)-1:K=K+1

100 if prmdiv(A)=A then print I, K:goto 120

105 if K>1000 then print I, 0:goto 120

110 H=A:goto 40

120 K=0

130 next

CROSSREFS

Sequence in context: A250304 A256916 A164705 * A193229 A112458 A019252

Adjacent sequences:  A073751 A073752 A073753 * A073755 A073756 A073757

KEYWORD

easy,nonn,base

AUTHOR

Felice Russo, Sep 02 2002

EXTENSIONS

More terms from Jason Earls, Sep 04 2002

STATUS

approved

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Last modified December 15 20:10 EST 2018. Contains 318154 sequences. (Running on oeis4.)