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A218030 Numbers k equal to half of the product of the nonzero (base-10) digits of k^2. 2
2, 5, 54, 648, 2160, 337169025526136832000, 685506275314921762068267522458966662115416623590907309075726336000000, 46641846972427276691124922228108091690332947069125333309512419901440000000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The first 5 terms of the sequence were found by the author around 1980 using his Commodore PET computer. He found the subsequent terms in 1991 by means of an improved program. The author has always referred to these as the "Faithy numbers" after his mother Faith who posed the problem.

LINKS

Michael S. Branicky, Table of n, a(n) for n = 1..12 (all terms < 10^300)

Michael S. Branicky, Python program

Giovanni Resta, C program for this and related sequences

EXAMPLE

For n=5, n^2 is 25; the product of the digits of 25 is 2*5 = 10, which is equal to 2*n.

MATHEMATICA

mx = 2^255; L = {};

p2 = 1; While[p2 < mx, Print["--> 2^", Log[2, p2]];

p3 = p2; While [p3 < mx,

p5 = p3; While[p5 < mx,

n = p5; While[n < mx,

If[2 n == Times @@ Select[IntegerDigits[n^2], # > 0 &],

AppendTo[L, n]; Print[n]]; n *= 7]; p5 *= 5]; p3 *= 3];

p2 *= 2]; Sort[L] (* Giovanni Resta, Oct 19 2012 *)

PROG

(PARI) is_A218030(n)={my(d=digits(n^2)); n*=2; for(i=1, #d, d[i]||next; n%d[i]&return; n\=d[i]); n==1} \\ M. F. Hasler, Oct 19 2012

CROSSREFS

Special case of A218013 where the ratio of the digit-product to the original number is 2. Related to A218072.

Sequence in context: A206848 A081482 A134475 * A114029 A013171 A073422

Adjacent sequences: A218027 A218028 A218029 * A218031 A218032 A218033

KEYWORD

nonn,base

AUTHOR

Nels Olson, Oct 18 2012

STATUS

approved

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Last modified December 9 02:22 EST 2022. Contains 358698 sequences. (Running on oeis4.)