login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A344366 Integers k such that the sum of squares of digits of both k and k-1 are prime. 0
12, 102, 111, 120, 160, 230, 250, 380, 410, 450, 520, 560, 720, 780, 830, 870, 1002, 1011, 1020, 1060, 1100, 1101, 1110, 1370, 1640, 1680, 1910, 1950, 1970, 1990, 2030, 2050, 2340, 2670, 2920, 3080, 3170, 3240, 3420, 3460, 3550, 3570, 3710, 3840, 3860, 4010 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Integers k such that k and k-1 are both in A108662.

Terms are never prime. They cannot end in the digits 3,4,5,6,7,8,9.

If k is a term, phi(k) is divisible by 4.

The set of such numbers is infinite.

LINKS

Table of n, a(n) for n=1..46.

Charles U. Lonappan, Consecutive natural numbers whose sum of squares of digits is prime, International Journal of Research in Engineering and Science (IJRES), Volume 9 Issue 1 (2021) pp. 14-16.

EXAMPLE

12 is in the sequence because the sum of squares of digits of 12 is 5 and that of 11 is 2, and both 5 and 2 are prime numbers.

MATHEMATICA

q[n_] := PrimeQ[Plus @@ (IntegerDigits[n]^2)]; Select[Range[2, 5000], q[#-1] && q[#] &] (* Amiram Eldar, May 19 2021 *)

PROG

(PARI) isok(k) = isprime(norml2(digits(k-1))) && isprime(norml2(digits(k))); \\ Michel Marcus, May 24 2021

CROSSREFS

Cf. A108662.

Sequence in context: A008547 A109020 A099299 * A304504 A344279 A022736

Adjacent sequences:  A344363 A344364 A344365 * A344367 A344368 A344369

KEYWORD

nonn,base

AUTHOR

Charles U. Lonappan, May 19 2021

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 24 20:26 EDT 2021. Contains 346273 sequences. (Running on oeis4.)