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 A049121 If a(n) is prime, a(n+1) = a(n) followed by a 1; if a(n) is composite, a(n+1) = concatenation of prime factors of a(n) (with multiplicity). 3
 0, 1, 11, 111, 337, 3371, 33711, 317661, 3195573, 33134361, 313849599, 3714945219, 33412771691, 1341179350213, 7181173232711, 28979247806109, 3318731171903071, 191053757165759337, 3505114323880284323, 131572281752894106563, 1091640386973585393403 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..60 EXAMPLE a(3)=111=3*37, so a(4)=3_37=337. 337 is prime, so a(5)=337_1=3371. MAPLE a:= proc(n) option remember; `if`(n=0, 0, (t-> parse(cat(`if`(isprime(t) or t<2, [t, 1][], sort(map(i-> i[1]\$i[2], ifactors(t)[2]))[]))))(a(n-1))) end: seq(a(n), n=0..21); # Alois P. Heinz, Apr 04 2022 MATHEMATICA nxt[n_]:=If[n==0||n==1||PrimeQ[n], FromDigits[Join[IntegerDigits[n], {1}]], FromDigits[Flatten[IntegerDigits/@(Table[#[[1]], {#[[2]]}]&/@ FactorInteger[n])]]]; NestList[nxt, 0, 20] (* Harvey P. Dale, May 02 2012 *) PROG (Python) from itertools import count, islice from sympy import isprime, factorint def agen(): yield from [0, 1] an = 11 for n in count(3): yield an if isprime(an): an = 10*an + 1 else: an = int("".join(str(p) for p in factorint(an, multiple=True))) print(list(islice(agen(), 21))) # Michael S. Branicky, Apr 04 2022 CROSSREFS Cf. A049201, A037274, A055657. Sequence in context: A209233 A062095 A055657 * A143573 A248039 A244204 Adjacent sequences: A049118 A049119 A049120 * A049122 A049123 A049124 KEYWORD nonn,easy,base,nice AUTHOR Mark Alexiuk (alexiuk(AT)ee.umanitoba.ca) EXTENSIONS More terms from Michel ten Voorde Corrected by Harvey P. Dale, Jun 06 2000. STATUS approved

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Last modified September 23 03:48 EDT 2023. Contains 365532 sequences. (Running on oeis4.)