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A094830
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Start with x = n, repeatedly replace x with x + sum of squares of digits of x until you reach a prime; sequence gives number of steps.
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5
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1, 0, 0, 6, 0, 4, 0, 11, 5, 1, 0, 1, 0, 1, 1, 1, 0, 1, 0, 5, 13, 9, 0, 4, 11, 12, 17, 3, 0, 8, 0, 8, 7, 1, 7, 3, 0, 5, 7, 4, 0, 3, 0, 3, 7, 8, 0, 2, 2, 2, 6, 3, 0, 10, 2, 3, 1, 3, 0, 3, 0, 2, 2, 18, 2, 11, 0, 2, 6, 9, 0, 10, 0, 1, 1, 2, 5, 1, 0, 16, 2, 8, 0, 3, 11, 6, 10, 2, 0, 4, 1, 15, 2, 1, 9, 2, 0, 7, 7, 1
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OFFSET
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1,4
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COMMENTS
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It is currently not known if this sequence is well defined, i.e. if for all n>=1 there exists an integer v such that a(n) = v. If the sequence is not well defined then the given programs are incorrect as they will get stuck in an infinite loop for some integers. - Peter Luschny, Oct 28 2016
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LINKS
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Robert Israel, Table of n, a(n) for n = 1..10000
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EXAMPLE
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a(4)=6 because 4 -> 20 -> 24 -> 44 -> 76 -> 161 -> 199, takes 6 steps to reach a prime.
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MAPLE
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f:= proc(n) local x, k;
x:= n;
for k from 0 do
if isprime(x) then return k fi;
x:= x + add(t^2, t = convert(x, base, 10))
od;
end proc:
map(f, [$1..100]); # Robert Israel, Oct 27 2016
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MATHEMATICA
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p[n_]:=Length[NestWhileList[#+Total[IntegerDigits[#]^2]&, n, !PrimeQ[ #]&]]-1; Array[p, 100] (* Harvey P. Dale, Dec 03 2011 *)
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PROG
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a(n) = my(k=0); while(!isprime(n), d=digits(n); n+=vecsum(vector(#d, i, d[i]^2)); k++) ; k \\ David A. Corneth, Oct 27 2016
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CROSSREFS
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Cf. A003132, A094877.
Sequence in context: A089841 A202542 A019766 * A196878 A209835 A298528
Adjacent sequences: A094827 A094828 A094829 * A094831 A094832 A094833
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KEYWORD
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base,easy,nonn,look
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AUTHOR
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Jason Earls, Jun 13 2004
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STATUS
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approved
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