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 A031149 Numbers n such that n^2 with last digit deleted is still a perfect square. 21
 0, 1, 2, 3, 4, 7, 13, 16, 19, 38, 57, 136, 253, 487, 604, 721, 1442, 2163, 5164, 9607, 18493, 22936, 27379, 54758, 82137, 196096, 364813, 702247, 870964, 1039681, 2079362, 3119043, 7446484, 13853287, 26666893, 33073696, 39480499, 78960998, 118441497, 282770296 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Square root of A023110(n). For the first 4 terms, the square has only one digit, but in analogy to the sequences in other bases (A204502, A204512,  A204514, A204516, A204518, A204520, A004275, A055793, A055792), it is understood that deleting this digit yields 0. From Robert Israel, Feb 16 2016: (Start) Solutions x to x^2 = 10*y^2 + j, j in {0,1,4,6,9}, in increasing order of x. j=0 occurs only for x=0. Let M be the 2 X 2 matrix [19, 60; 6, 19]. Solutions of x^2 = 10*y^2 + 1 are (x,y)^T = M^k (1,0)^T for k >= 0. Solutions of x^2 = 10*y^2 + 4 are (x,y)^T = M^k (2,0)^T for k >= 0. Solutions of x^2 = 10*y^2 + 6 are (x,y)^T = M^k (4,1)^T and M^k (16,5)^T for k >= 0. Solutions of x^2 = 10*y^2 + 9 are (x,y)^T = M^k (3,0)^T, M^k (7,2)^T, M^k (13,4)^T for k >= 0. Since (1,0)^T <= (2,0)^T <= (3,0)^T <= (4,1)^T <= (7,2)^T <= (13,4)^T <= (16,5)^T <= (19,6)^T = M (1,0)^T (element-wise) and M has positive entries, we see that the terms always occur in this order, for successive k. The eigenvalues of M are 19 + 6*sqrt(10) and 19 - 6*sqrt(10). From this follow my formulas below and the G.f. (End) REFERENCES R. K. Guy, Neg and Reg, preprint, Jan 2012. [From N. J. A. Sloane, Jan 12 2012] LINKS Dmitry Petukhov and Robert Israel, Table of n, a(n) for n = 1..4400 (n = 1..67 from Dmitry Petukhov) M. F. Hasler, Truncated squares, OEIS wiki, Jan 16 2012 FORMULA Appears to satisfy: a(n)=38a(n-7)-a(n-14) which would require a(-k) to look like 3, 2, 1, 4, 7, 13, 16, 57, 38, 19, 136, ... for k>0. - Henry Bottomley, May 08 2001 Empirical g.f.: x^2*(1 + 2*x + 3*x^2 + 4*x^3 + 7*x^4 + 13*x^5 + 16*x^6 - 19*x^7 - 38*x^8 - 57*x^9 - 16*x^10 - 13*x^11 - 7*x^12 - 4*x^13) / ((1 - 38*x^7 + x^14)). - Colin Barker, Jan 17 2014 a(n) = 38*a(n-7) - a(n-14) for n>15 (conjectured). - Colin Barker, Dec 31 2017 With e1 = 19 + 6*sqrt(10) and e2 = 19 - 6*sqrt(10),    a(2+7k) = (e1^k + e2^k)/2,    a(3+7k) = e1^k + e2^k,    a(4+7k) = (3/2) (e1^k + e2^k),    a(5+7k) = (2+sqrt(10)/2) e1^k + (2-sqrt(10)/2) e2^k,    a(6+7k) = (7/2+sqrt(10)) e1^k + (7/2-sqrt(10)) e2^k,    a(7+7k) = (13/2+2 sqrt(10)) e1^k + (13/2-2 sqrt(10)) e2^k, a(8+7k) = (8+5 sqrt(10)/2) e1^k + (8-5 sqrt(10)/2) e2^k. - Robert Israel, Feb 16 2016 EXAMPLE 364813^2 = 133088524969, 115364^2 = 13308852496. MAPLE for i from 1 to 150000 do if (floor(sqrt(10 * i^2 + 9)) > floor(sqrt(10 * i^2))) then print(floor(sqrt(10 * i^2 + 9))) end if end do; MATHEMATICA fQ[n_] := IntegerQ@ Sqrt@ Quotient[n^2, 10]; Select[ Range[ 0, 40000000], fQ] (* Harvey P. Dale, Jun 15 2011 *) (* modified by Robert G. Wilson v, Jan 16 2012 *) PROG (PARI) s=[]; for(n=0, 1e7, if(issquare(n^2\10), s=concat(s, n))); s \\ Colin Barker, Jan 17 2014; typo fixed by Zak Seidov, Jan 31 2014 CROSSREFS Cf. A023110, A030686, A030687, A031150. Sequence in context: A057983 A200088 A004783 * A096723 A266498 A137495 Adjacent sequences:  A031146 A031147 A031148 * A031150 A031151 A031152 KEYWORD nonn,base AUTHOR EXTENSIONS 4 initial terms added by M. F. Hasler, Jan 15 2012 a(40) from Robert G. Wilson v, Jan 15 2012 STATUS approved

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Last modified October 22 18:50 EDT 2019. Contains 328319 sequences. (Running on oeis4.)