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A073756 Number of steps needed to reach a prime when the following map is repeatedly applied to n: if n is even then 2n + int(log(n)) + 1, otherwise 2n - int(log(n)) - 1; or -1 if no prime is ever reached. 5
1, 3, 2, 2, 2, 19, 1, 19, 1, 1, 18, 1, 1, 18, 3, 1, 12, 3, 1, 9, 15, 3, 8, 3, 3, 17, 77, 3, 5, 6, 3, 9, 2, 2, 2, 31, 8, 11, 2, 5, 2, 11, 69, 34, 2, 17, 14, 2, 16, 33, 7, 8, 2, 14, 2, 1, 5, 1, 76, 2, 8, 5, 4, 76, 1, 8, 2, 4, 30, 1, 1, 2, 10, 30, 1, 1, 4, 10, 7, 1, 10, 4, 1, 7, 33, 10, 1, 1, 16, 33 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

LINKS

Table of n, a(n) for n=2..91.

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+int(log(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-int(log(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: A239736 A065437 A097721 * A328829 A006379 A217956

Adjacent sequences:  A073753 A073754 A073755 * A073757 A073758 A073759

KEYWORD

easy,nonn

AUTHOR

Felice Russo, Sep 02 2002

STATUS

approved

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Last modified August 4 14:04 EDT 2020. Contains 336201 sequences. (Running on oeis4.)