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a(n) = n if n is not an odd prime number. Otherwise, a(n) = k*floor(n/10), where k is the smallest integer such that n < 10^k.
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%I #10 Aug 04 2019 01:38:42

%S 0,1,2,0,4,0,6,0,8,9,10,2,12,2,14,15,16,2,18,2,20,21,22,4,24,25,26,27,

%T 28,4,30,6,32,33,34,35,36,6,38,39,40,8,42,8,44,45,46,8,48,49,50,51,52,

%U 10,54,55,56,57,58,10,60,12,62,63,64,65,66,12,68

%N a(n) = n if n is not an odd prime number. Otherwise, a(n) = k*floor(n/10), where k is the smallest integer such that n < 10^k.

%p f:= proc(n) local k;

%p if n::odd and isprime(n) then

%p k:= ilog10(n)+1;

%p k*floor(n/10)

%p else n

%p fi

%p end proc:

%p map(f, [$0..100]); # _Robert Israel_, Aug 04 2019

%Y Cf. A131516, A130758, A000027, A001477, A006005.

%K nonn

%O 0,3

%A _Mohammad K. Azarian_, Aug 21 2007

%E Offset changed by _Mohammad K. Azarian_, Nov 19 2008

%E Offset corrected by _Robert Israel_, Aug 04 2019