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 A309521 The sequence is always extended by subtracting the n-th digit of the sequence from a(n) if a(n) is prime, else adding it to a(n), starting with a(1) = 4. 3
 4, 8, 16, 17, 11, 10, 17, 16, 17, 16, 16, 17, 10, 11, 5, 4, 11, 10, 16, 17, 11, 10, 17, 16, 16, 17, 16, 21, 25, 26, 27, 28, 28, 29, 23, 22, 29, 28, 29, 28, 28, 29, 22, 23, 17, 16, 22, 23, 16, 17, 11, 9, 10, 12, 17, 15, 21, 23, 16, 18, 26, 28, 36, 38, 47, 45 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Up to 10^6 the terms increase approximately linearly, and no cycles are visible. As we approach a million terms the terms are 3590238, 3590240, 3590242, 3590248, 3590251, 3590251, 3590257, 3590259, 3590261, 3590267, 3590270, 3590279, 3590285, 3590287, 3590289, 3590295, 3590298, 3590306, 3590312, 3590314, 3590316, 3590322, 3590326, 3590326, 3590332, 3590334, 3590336, 3590342, 3590346, ..., and the smallest values not yet encountered at that point are 2, 3, 6, 7, 13, 14, 19, 20, 24, 30, 31, 32, 33, 34, 35, 37, 39, 40, 41, 42, 43, 44, 46, 49, 51, 53, 55, 56, 57, 58, 59, 60, 61, 66, 67, 68, 69, 70, 75, 76, 77, 78, 79, 80, 81, 83, 85, 86, 87, 88, 89, 90, 91, 92, 100, 108, 109, 110, 113, 114, 115, 116, 117, 118, ... - Lars Blomberg, Aug 06 2019 LINKS Lars Blomberg, Table of n, a(n) for n = 1..10000 EXAMPLE The sequence starts with 4,8,16,17,11,10,17,16,... As a(1) = 4 (not a prime), we have a(2) = a(1) + [the 1st digit of the seq] = 4 + 4 = 8; as a(2) = 8 (not a prime), we have a(3) = a(2) + [the 2nd digit of the seq] = 8 + 8 = 16; as a(3) = 16 (not a prime), we have a(4) = a(3) + [the 3rd digit of the seq] = 16 + 1 = 17; as a(4) = 17 (a prime), we have a(5) = a(4) - [the 4th digit of the seq] = 17 - 6 = 11; as a(5) = 11 (a prime), we have a(6) = a(5) - [the 5th digit of the seq] = 11 - 1 = 10; etc. PROG (PARI) v=4; d=[]; for (n=1, 66, print1 (v ", "); d=concat(d, digits(v)); v+=d[n]*if (isprime(v), -1, +1)) \\ Rémy Sigrist, Aug 06 2019 CROSSREFS Cf. A309529 (same idea, but dealing with even numbers instead of primes). Sequence in context: A166634 A189007 A242349 * A072603 A123535 A312765 Adjacent sequences: A309518 A309519 A309520 * A309522 A309523 A309524 KEYWORD base,nonn AUTHOR Eric Angelini and Lars Blomberg, Aug 06 2019 EXTENSIONS More terms from Rémy Sigrist, Aug 06 2019 STATUS approved

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Last modified July 19 10:32 EDT 2024. Contains 374392 sequences. (Running on oeis4.)