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A239639 Lengths of runs of the initial digits of semiprimes in decimal representation, cf. A239634.

%I A239639 #7 Jun 02 2025 09:30:05
%S A239639 1,1,1,3,4,5,2,4,3,2,4,4,28,32,32,27,33,26,30,28,29,278,264,264,260,
%T A239639 257,245,262,252,244,2456,2380,2329,2320,2254,2325,2231,2244,2214,
%U A239639 21852,21400,20923,20773,20548,20528,20258,20156,20219,197249,193162,190540,188288,187308,185657,184788,183856,183441
%N A239639 Lengths of runs of the initial digits of semiprimes in decimal representation, cf. A239634.
%e A239639 .   n | a(n) | runs in A001358 having a common initial digit
%e A239639 . ----+------+-------------------------------------------------------
%e A239639 .   1 |    1 | [4]
%e A239639 .   2 |    1 | [6]
%e A239639 .   3 |    1 | [9]
%e A239639 .   4 |    3 | [10, 14, 15]
%e A239639 .   5 |    4 | [21, 22, 25, 26]
%e A239639 .   6 |    5 | [33, 34, 35, 38, 39]
%e A239639 .   7 |    2 | [46, 49]
%e A239639 .   8 |    4 | [51, 55, 57, 58]
%e A239639 .   9 |    3 | [62, 65, 69]
%e A239639 .  10 |    2 | [74, 77]
%e A239639 .  11 |    4 | [82, 85, 86, 87]
%e A239639 .  12 |    4 | [91, 93, 94, 95]
%e A239639 .  13 |   28 | [106,111,115,118,119,121,122,123,129,133,134,141,
%e A239639 .     |      |  142,143,145,146,155,158,159,161,166,169,177,178,
%e A239639 .     |      |  183,185,187,194,201]
%e A239639 .  14 |   32 | [201,202,203,205,206,209,213,214,215,217,218,219,
%e A239639 .     |      |  221,226,235,237,247,249,253,254,259,262,265,267,
%e A239639 .     |      |  274,278,287,289,291,295,298,299]
%e A239639 .  15 |   32 | [301,302,303,305,309,314,319,321,323,326,327,329,
%e A239639 .     |      |  334,335,339,341,346,355,358,361,362,365,371,377,
%e A239639 .     |      |  381,382,386,391,393,394,395,398]
%e A239639 .  16 |   27 | [403,407,411,413,415,417,422,427,437,445,446,447,
%e A239639 .     |      |  451,453,454,458,466,469,471,473,478,481,482,485,
%e A239639 .     |      |  489,493,497].
%o A239639 (Haskell)
%o A239639 import Data.List (group)
%o A239639 a239639 n = a239639_list !! (n-1)
%o A239639 a239639_list = map length $ group a239634_list
%Y A239639 Cf. A001358.
%K A239639 nonn,base
%O A239639 1,4
%A A239639 _Reinhard Zumkeller_, Mar 23 2014