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 A098731 Numbers k such that 2*R_k is a happy number (A007770), where R_k = 11...1 is the repunit (A002275) of length k. 1
 7, 8, 11, 17, 25, 44, 47, 48, 52, 59, 70, 80, 89, 92, 94, 98, 101, 110, 116, 124, 134, 139, 152, 158, 161, 164, 170, 175, 179, 184, 187, 196, 205, 209, 215, 222, 226, 228, 233, 235, 241, 250, 269, 272, 273, 278, 282, 287, 288, 296, 297, 303, 322, 325, 333, 361 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Chai Wah Wu, Table of n, a(n) for n = 1..10000 EXAMPLE 2*R_11 = 2*11111111111 = 22222222222 is a happy number, so 11 is in the list. PROG (Python) from itertools import count, islice def A098731_gen(startvalue=1): # generator of terms for n in count(max(startvalue, 1)): m = n<<2 while m not in {1, 37, 58, 89, 145, 42, 20, 4, 16}: m = sum((0, 1, 4, 9, 16, 25, 36, 49, 64, 81)[ord(d)-48] for d in str(m)) if m == 1: yield n A098731_list = list(islice(A098731_gen(), 20)) # Chai Wah Wu, Aug 02 2023 CROSSREFS Cf. A007770, A002275. Sequence in context: A102963 A117619 A226977 * A348784 A294483 A105740 Adjacent sequences: A098728 A098729 A098730 * A098732 A098733 A098734 KEYWORD easy,nonn,base AUTHOR Lambert Klasen (lambert.klasen(AT)gmx.de), Oct 28 2004 STATUS approved

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Last modified September 21 18:26 EDT 2023. Contains 365503 sequences. (Running on oeis4.)