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 A348831 Positive numbers whose square starts and ends with exactly 44, and no 444. 1
 212, 2112, 6638, 6662, 6688, 20988, 21012, 21062, 21112, 21138, 21162, 21188, 21212, 66338, 66362, 66388, 66412, 66438, 66488, 66512, 66562, 66588, 66612, 66712, 66738, 66762, 66788, 66812, 66838, 66862, 66888, 66912, 66938, 66988, 67012, 67062, 209762, 209788 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS When a square starts and ends with digits dd, then dd is necessarily 44. The last 2 digits of terms are either 12, 38, 62 or 88. From Marius A. Burtea, Nov 09 2021 : (Start) The sequence is infinite because the numbers 212, 2112, 21112, ..., (19*10^k + 8) / 9, k >= 3, are terms because the remainder when dividing by 1000 is 544 and 445*10^(2*k - 2) < ((19*10^k + 8) / 9)^2 < 447*10^(2*k - 2), k >= 3. Also 6638, 66338, 663338, 6633338, 66333338, 663333338, 6633333338, ..., (199*10^k + 14) / 3, k >= 2, are terms and have no digits 0, because their squares are: 44063044, 4400730244, 4400730244, 440017302244, 44001173022244, 4400111730222244, 440011117302222244, ... (End) LINKS Table of n, a(n) for n=1..38. EXAMPLE 212 is a term since 212^2 = 44944. 662 is not a term since 662^2 = 438244. 668 is not a term since 668^2 = 446224. 2108 is not a term since 2108^2 = 4443664. 21038 is not a term since 21038^2 = 442597444. 21088 is not a term since 21088^2 = 444703744. MATHEMATICA Select[Range[10, 300000], (d = IntegerDigits[#^2])[[1 ;; 2]] == d[[-2 ;; -1]] == {4, 4} && d[[-3]] != 4 && d[[3]] != 4 &] (* Amiram Eldar, Nov 08 2021 *) PROG (Magma) fd:=func; fs:=func; [n:n in [1..210000]|fd(n) and fs(n)]; // Marius A. Burtea, Nov 08 2021 (Python) from itertools import count, takewhile def ok(n): s = str(n*n); return len(s.rstrip("4")) == len(s.lstrip("4")) == len(s)-2 def aupto(N): ends = [12, 38, 62, 88] r = takewhile(lambda x: x<=N, (100*i+d for i in count(0) for d in ends)) return [k for k in r if ok(k)] print(aupto(209788)) # Michael S. Branicky, Nov 08 2021 CROSSREFS Cf. A017317. Subsequence of A045858, A273375, A305719 and A346774. Similar to: A348488 (d=4), this sequence (dd=44), A348832 (ddd=444). Sequence in context: A235180 A353139 A114880 * A200433 A222950 A356358 Adjacent sequences: A348828 A348829 A348830 * A348832 A348833 A348834 KEYWORD nonn,base AUTHOR Bernard Schott, Nov 08 2021 STATUS approved

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Last modified June 17 13:47 EDT 2024. Contains 373445 sequences. (Running on oeis4.)