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%I #32 Jan 09 2021 22:45:38
%S 0,1,3,6,2,7,13,20,12,21,11,22,10,23,9,24,8,25,43,62,42,63,41,64,40,
%T 15,41,14,42,71,101,132,100,67,33,68,32,69,31,70,30,71,29,72,28,73,27,
%U 74,26,75,125,176,124,177,123,178,122,179,121,180,120,181,119,56,120,55,121,188,256,325,255
%N a(0) = 0, a(1) = 1; for n > 1, a(n) = a(n-1) - n if a(n) is nonnegative, not already in the sequence, and gcd(a(n-1),n) > 1 or gcd(a(n-2),n) > 1. Otherwise a(n) = a(n-1) + n.
%C This sequence is a variation of the Recamán sequence A005132 where the same rules apply except an additional restriction is added whereby a(n) = a(n-1) - n can occur only if gcd(a(n-1),n) > 1 or gcd(a(n-2),n) > 1, where gcd is the greatest common divisor. This additional restriction is inspired by the selection rules of A336957 and A098550.
%C Initially the sequence terms show a similar pattern to the Recamán sequence. However after about 1.5 million terms they begin to predominantly oscillate between two or a small number of values and the pattern of arching lines is no longer present. See the linked images.
%C It is unclear if all values are eventually visited; numerous small values like 4 and 5 have not occurred after 50 million terms.
%H Scott R. Shannon, <a href="/A339192/a339192_4.png">Image of the terms for n=0 to 10000</a>. The values form a pattern very similar to the Recamán sequence.
%H Scott R. Shannon, <a href="/A339192/a339192.png">Image of the terms for n=0 to 2000000</a>. Notice the change in behavior after about 1.5 million terms.
%H Scott R. Shannon, <a href="/A339192/a339192_1.png">Image of the terms for n=0 to 10000000</a>.
%H Scott R. Shannon, <a href="/A339192/a339192_2.png">Image of the terms for n=0 to 50000000</a>.
%e a(4) = 2. As gcd(a(3),4) = gcd(6,4) = 2 > 1, and as 6 - 4 = 2 has not occurred previously, a(4) = 2.
%e a(23) = 64. a(22) = 41, and 41 - 23 = 18 has not occurred previously. However as gcd(41,23) = 1 and gcd(a(21),23) = gcd(63,23) = 1, both additional criteria for subtraction fail, thus a(23) = a(22) + 23 = 41 + 23 = 64. This is the first term that differs from the standard Recamán sequence A005132.
%e a(57) = 179. a(56) = 122, and 122 - 57 = 65 has not occurred previously. However as gcd(122,57) = 1 and gcd(a(55),57) = gcd(178,57) = 1, both additional criteria for subtraction fail, thus a(57) = a(56) + 57 = 122 + 57 = 179. This is the first term where n is a composite, less than the last term, and a(n-1) - n is available, but due to the gcd requirements the next term is forced to be a(n-1) + n.
%Y Cf. A339557, A339670, A339671, A005132, A336957, A098550.
%K nonn
%O 0,3
%A _Scott R. Shannon_, Dec 07 2020
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