{"group":{"id":1,"name":"Community","lockable":false,"created_at":"2012-01-18T18:02:15.000Z","updated_at":"2026-05-26T00:16:20.000Z","description":"Problems submitted by members of the MATLAB Central community.","is_default":true,"created_by":161519,"badge_id":null,"featured":false,"trending":false,"solution_count_in_trending_period":0,"trending_last_calculated":"2026-05-26T00:00:00.000Z","image_id":null,"published":true,"community_created":false,"status_id":2,"is_default_group_for_player":false,"deleted_by":null,"deleted_at":null,"restored_by":null,"restored_at":null,"description_opc":null,"description_html":null,"published_at":null},"problems":[{"id":44385,"title":"Extra safe primes","description":"Did you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\r\n\r\nTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is *also a safe prime*.\r\n\r\n*Examples*\r\n\r\n  isextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\r\n  isextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)\r\n","description_html":"\u003cp\u003eDid you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\u003c/p\u003e\u003cp\u003eTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is \u003cb\u003ealso a safe prime\u003c/b\u003e.\u003c/p\u003e\u003cp\u003e\u003cb\u003eExamples\u003c/b\u003e\u003c/p\u003e\u003cpre class=\"language-matlab\"\u003eisextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\r\nisextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)\r\n\u003c/pre\u003e","function_template":"function tf = isextrasafe(x)\r\n    tf = false;\r\nend","test_suite":"%%\r\nx = 0;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 5;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 7;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 11;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 15;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 23;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 71;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 719;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 2039;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 2040;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 5807;\r\nassert(isequal(isextrasafe(x),true))","published":true,"deleted":false,"likes_count":13,"comments_count":4,"created_by":4793,"edited_by":null,"edited_at":null,"deleted_by":null,"deleted_at":null,"solvers_count":761,"test_suite_updated_at":"2017-10-19T17:09:19.000Z","rescore_all_solutions":true,"group_id":34,"created_at":"2017-10-13T20:02:13.000Z","updated_at":"2026-05-28T04:55:19.000Z","published_at":"2017-10-16T01:45:10.000Z","restored_at":null,"restored_by":null,"spam":false,"simulink":false,"admin_reviewed":false,"description_opc":"{\"relationships\":[{\"relationshipType\":\"http://schemas.mathworks.com/matlab/code/2013/relationships/document\",\"targetMode\":\"\",\"relationshipId\":\"rId1\",\"target\":\"/matlab/document.xml\"},{\"relationshipType\":\"http://schemas.mathworks.com/matlab/code/2013/relationships/output\",\"targetMode\":\"\",\"relationshipId\":\"rId2\",\"target\":\"/matlab/output.xml\"}],\"parts\":[{\"partUri\":\"/matlab/document.xml\",\"relationship\":[],\"contentType\":\"application/vnd.mathworks.matlab.code.document+xml\",\"content\":\"\u003c?xml version=\\\"1.0\\\" encoding=\\\"UTF-8\\\"?\u003e\\n\u003cw:document xmlns:w=\\\"http://schemas.openxmlformats.org/wordprocessingml/2006/main\\\"\u003e\u003cw:body\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003eDid you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003eTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is\u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:t\u003e \u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:rPr\u003e\u003cw:b/\u003e\u003c/w:rPr\u003e\u003cw:t\u003ealso a safe prime\u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:t\u003e.\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:rPr\u003e\u003cw:b/\u003e\u003c/w:rPr\u003e\u003cw:t\u003eExamples\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"code\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003e\u003c![CDATA[isextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\\nisextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)]]\u003e\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003c/w:body\u003e\u003c/w:document\u003e\"},{\"partUri\":\"/matlab/output.xml\",\"contentType\":\"text/xml\",\"content\":\"\u003c?xml version=\\\"1.0\\\" encoding=\\\"UTF-8\\\" standalone=\\\"no\\\" ?\u003e\u003cembeddedOutputs\u003e\u003cmetaData\u003e\u003cevaluationState\u003emanual\u003c/evaluationState\u003e\u003clayoutState\u003ecode\u003c/layoutState\u003e\u003coutputStatus\u003eready\u003c/outputStatus\u003e\u003c/metaData\u003e\u003coutputArray type=\\\"array\\\"/\u003e\u003cregionArray type=\\\"array\\\"/\u003e\u003c/embeddedOutputs\u003e\"}]}"}],"problem_search":{"problems":[{"id":44385,"title":"Extra safe primes","description":"Did you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\r\n\r\nTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is *also a safe prime*.\r\n\r\n*Examples*\r\n\r\n  isextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\r\n  isextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)\r\n","description_html":"\u003cp\u003eDid you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\u003c/p\u003e\u003cp\u003eTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is \u003cb\u003ealso a safe prime\u003c/b\u003e.\u003c/p\u003e\u003cp\u003e\u003cb\u003eExamples\u003c/b\u003e\u003c/p\u003e\u003cpre class=\"language-matlab\"\u003eisextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\r\nisextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)\r\n\u003c/pre\u003e","function_template":"function tf = isextrasafe(x)\r\n    tf = false;\r\nend","test_suite":"%%\r\nx = 0;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 5;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 7;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 11;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 15;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 23;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 71;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 719;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 2039;\r\nassert(isequal(isextrasafe(x),true))\r\n\r\n%%\r\nx = 2040;\r\nassert(isequal(isextrasafe(x),false))\r\n\r\n%%\r\nx = 5807;\r\nassert(isequal(isextrasafe(x),true))","published":true,"deleted":false,"likes_count":13,"comments_count":4,"created_by":4793,"edited_by":null,"edited_at":null,"deleted_by":null,"deleted_at":null,"solvers_count":761,"test_suite_updated_at":"2017-10-19T17:09:19.000Z","rescore_all_solutions":true,"group_id":34,"created_at":"2017-10-13T20:02:13.000Z","updated_at":"2026-05-28T04:55:19.000Z","published_at":"2017-10-16T01:45:10.000Z","restored_at":null,"restored_by":null,"spam":false,"simulink":false,"admin_reviewed":false,"description_opc":"{\"relationships\":[{\"relationshipType\":\"http://schemas.mathworks.com/matlab/code/2013/relationships/document\",\"targetMode\":\"\",\"relationshipId\":\"rId1\",\"target\":\"/matlab/document.xml\"},{\"relationshipType\":\"http://schemas.mathworks.com/matlab/code/2013/relationships/output\",\"targetMode\":\"\",\"relationshipId\":\"rId2\",\"target\":\"/matlab/output.xml\"}],\"parts\":[{\"partUri\":\"/matlab/document.xml\",\"relationship\":[],\"contentType\":\"application/vnd.mathworks.matlab.code.document+xml\",\"content\":\"\u003c?xml version=\\\"1.0\\\" encoding=\\\"UTF-8\\\"?\u003e\\n\u003cw:document xmlns:w=\\\"http://schemas.openxmlformats.org/wordprocessingml/2006/main\\\"\u003e\u003cw:body\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003eDid you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003eTo celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is\u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:t\u003e \u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:rPr\u003e\u003cw:b/\u003e\u003c/w:rPr\u003e\u003cw:t\u003ealso a safe prime\u003c/w:t\u003e\u003c/w:r\u003e\u003cw:r\u003e\u003cw:t\u003e.\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"text\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:rPr\u003e\u003cw:b/\u003e\u003c/w:rPr\u003e\u003cw:t\u003eExamples\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003cw:p\u003e\u003cw:pPr\u003e\u003cw:pStyle w:val=\\\"code\\\"/\u003e\u003c/w:pPr\u003e\u003cw:r\u003e\u003cw:t\u003e\u003c![CDATA[isextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime\\nisextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)]]\u003e\u003c/w:t\u003e\u003c/w:r\u003e\u003c/w:p\u003e\u003c/w:body\u003e\u003c/w:document\u003e\"},{\"partUri\":\"/matlab/output.xml\",\"contentType\":\"text/xml\",\"content\":\"\u003c?xml version=\\\"1.0\\\" encoding=\\\"UTF-8\\\" standalone=\\\"no\\\" ?\u003e\u003cembeddedOutputs\u003e\u003cmetaData\u003e\u003cevaluationState\u003emanual\u003c/evaluationState\u003e\u003clayoutState\u003ecode\u003c/layoutState\u003e\u003coutputStatus\u003eready\u003c/outputStatus\u003e\u003c/metaData\u003e\u003coutputArray type=\\\"array\\\"/\u003e\u003cregionArray type=\\\"array\\\"/\u003e\u003c/embeddedOutputs\u003e\"}]}"}],"errors":[],"facets":[[{"value":"Cody5:Easy","count":1,"selected":false},{"value":"Number theory","count":1,"selected":false},{"value":"The Prime Directive","count":1,"selected":false}],[{"value":"medium","count":1,"selected":false}]],"term":"tag:\"safe prime\"","page":1,"per_page":50,"sort":"map(difficulty_value,0,0,999) asc"}}