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A073755 Number of steps needed to reach a prime when the following map is repeatedly applied to n: if n is even then 2n + int(sqrt(n)) + 1, otherwise 2n - int(sqrt(n)) - 1; or -1 if no prime is ever reached. 5
10, 2, 1, 1, 9, 1, 1, 4, 2, 2, 5, 28, 3, 8, 1, 1, 1, 17, 2, 1, 27, 1, 1, 34, 7, 2, 4, 12, 4, 3, 2, 16, 2, 2, 1, 1, 1, 1, 12, 4, 9, 1, 33, 1, 6, 12, 1, 26, 2, 16, 11, 5, 21, 4, 2, 2, 6, 8, 15, 2, 3, 6, 1, 11, 3, 27, 2, 4, 1, 15, 2, 1, 1, 3, 12, 2, 2, 1, 8, 2, 7, 3, 6, 3, 16, 11, 4, 2, 25, 8, 4, 10 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

LINKS

Table of n, a(n) for n=2..93.

EXAMPLE

For n=3, a(3)=2 because 3 -> 4 -> 11

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(sqrt(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(sqrt(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: A057603 A040097 A010174 * A010173 A136712 A138999

Adjacent sequences:  A073752 A073753 A073754 * A073756 A073757 A073758

KEYWORD

easy,nonn

AUTHOR

Felice Russo, Sep 02 2002

STATUS

approved

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Last modified December 20 06:03 EST 2014. Contains 252241 sequences.